2DEG Concentration Research Articles

Electric field dynamics in nitride structures containing quaternary alloy (Al, In, Ga)N

Molecular beam epitaxy growth and basic physical properties of quaternary AlInGaN layers, sufficiently thick for construction of electron blocking layers (EBL), embedded in ternary InGaN layers are presented. Transmission electron microscopy (TEM) measurement revealed good crystallographic structure and compositional uniformity of the quaternary layers contained in other nitride layers, which are typical for construction of nitride based devices. The AlInGaN layer was epitaxially compatible to InGaN matrix, strained, and no strain related dislocation creation was observed. The strain penetrated for limited depth, below 3 nm, even for relatively high content of indium (7%). For lower indium content (0.6%), the strain was below the detection limit by TEM strain analysis. The structures containing quaternary AlInGaN layers were studied by time dependent photoluminescence (PL) at different temperatures and excitation powers. It was shown that PL spectra contain three peaks: high energy donor bound exciton peak from the bulk GaN (DX GaN) and the two peaks (A and B) from InGaN layers. No emission from quaternary AlInGaN layers was observed. An accumulation of electrons on the EBL interface in high-In sample and formation of 2D electron gas (2DEG) was detected. The dynamics of 2DEG was studied by time resolved luminescence revealing strong dependence of emission energy on the 2DEG concentration. Theoretical calculations as well as power-dependence and temperature-dependence analysis showed the importance of electric field inside the structure. At the interface, the field was screened by carriers and could be changed by illumination. From these measurements, the dynamics of electric field was described as the discharge of carriers accumulated on the EBL.

Improved package reliability of AlGaN/GaN HFETs on 150 mm Si substrates by SiNx/polyimide dual passivation layers

Abstract In order to prevent reduction of output power of AlGaN/GaN heterostructure field effect transistors(HFETs) with inorganic passivation layer such as SiO 2 and SiN x after TO-220 plastic package, SiN x /photosensitive polyimide(PSPI) dual layers were proposed as passivation layers for high power, enhanced mode AlGaN/GaN HFETs on 6 in. Si substrates. The PSPI layer can endure the filler-induced mechanical stress during the plastic package, followed by prevention of the passivation layer cracking and metal deformation. Furthermore the SiN x layer can suppress the diffusion of fluorine ions during curing of the PSPI layer, which causes the reduction of 2DEG concentration due to the strong electronegativity of fluorine ions. The results showed that output power of the AlGaN/GaN HFETs with the SiN x /PSPI dual passivation layers was not deteriorated even after plastic package process, which can be attributed to prevention of the filler-induced stress and fluorine ion diffusion by the dual passivation layers.

Theoretical evaluation of two dimensional electron gas characteristics of quaternary AlxInyGa1–x–yN/GaN hetero-junctions

The two dimensional electron gas (2DEG) characteristics of gated metal-face wurtzite AlInGaN/GaN hetero-junctions including positions of subband energy levels, fermi energy level, and the 2DEG concentration as functions of physical and compositional properties of the hetero-junction (i.e., barrier thickness and metal mole-fractions) are theoretically evaluated using the variational method. The calculated values of the 2DEG concentration are in good agreement with the sparsely available experimental data reported in the literature. According to our simulation results, a considerable shift in the positive direction of threshold voltage of AlInGaN/GaN hetero-junction field-effect transistors can be achieved by engineering both the spontaneous and the piezoelectric polarizations using a quaternary AlInGaN barrier-layer of appropriate mole-fractions.

Influence of the additional p+ doped layers on the properties of AlGaAs/InGaAs/AlGaAs heterostructures for high power SHF transistors

The peculiarities of a new type of pseudomorphic AlGaAs/InGaAs/AlGaAs heterostructures with the additional acceptor doping of barriers used for the creation of the power SHF pseudomorphic high electron mobility transistor (pHEMT) have been studied. A comparison of the transport characteristic of the new and typical pHEMT heterostructures was carried out. The influence of the doped acceptor impurities in the AlGaAs barriers of the new pHEMT heterostructure on the transport properties was studied. It was shown that the application of the additional p+ doped barrier layers allows the achievement of a double multiplex increase in the two-dimensional electron gas (2DEG) concentration in the InGaAs quantum well with no parasite parallel conductivity in the AlGaAs barrier layers. An estimation of the concentration of the doped donors and acceptors penetrating into the deliberately undoped InGaAs quantum well from the AlGaAs barriers was performed by second ion mass spectrometry and photoluminescence spectrometry methods. Taking into account the electron scattering by the ionized impurity atoms, calculation of the electron mobility in the InGaAs channel showed that some reduction of the electron mobility results from scattering by the ionized Si donor due to an increase in the Si concentration and, therefore, is not caused by the application of additional p+ doped layers in the construction of pHEMT heterostructures.

Theoretical investigation of 2DEG concentration and mobility in the AlGaN/GaN heterostructures with various Al concentrations

AbstractThree‐scale model for the calculation of 2DEG mobility in AlGaN/GaN heterostructures developed by our group earlier was used for the investigation of the dependence of 2DEG concentration and mobility on the Al concentration in AlGaN layer. The model allows to calculate following 2DEG properties: energy levels, corresponding wavefunctions, potential energy distribution, charge carriers concentration distribution over the heterostructure. It is also possible to calculate electron mobility in 2DEG taking into account various scattering mechanisms. In the framework of this model the values of 2DEG concentration and mobility were calculated for various Al concentrations x in AlxGa1‐xN barrier (0.15 ≤ x ≤ 1). It is also taken into consideration that maximum barrier width should decrease with increasing Al concentration in order to allow pseudomorphic growth of barrier layer. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Piezoelectricity-Induced Schottky Barrier Height Variations in AlGaN/GaN High Electron Mobility Transistors

Drain current of AlGaN/GaN high electron mobility transistors (HEMTs) is measured to decrease/increase with in-plane tensile/compressive external strain. Such a trend is opposite to the conventional theory of direct piezoelectric effect on 2-D electron gas (2DEG). The reason is found to be the dependence of nickel gate barrier height on external strain, which strongly affects HEMTs’ threshold voltage and 2DEG concentration. The Ni/AlGaN interface states are proposed to be responsible for strain-induced gate barrier variations, which are important for device performances and sensor applications.

Trap states in enhancement-mode double heterostructures AlGaN/GaN high electron mobility transistors with different GaN channel layer thicknesses

This is the report on trap states in enhancement-mode AlGaN/GaN/AlGaN double heterostructures high electron mobility transistors by fluorine plasma treatment with different GaN channel layer thicknesses. Compared with the thick GaN channel layer sample, the thin one has smaller 2DEG concentration, lower electron mobility, lower saturation current, and lower peak transconductance, but it has a higher threshold voltage of 1.2 V. Deep level transient spectroscopy measurements are used to obtain the accurate capture cross section of trap states. By frequency dependent capacitance and conductance measurements, the trap state density of (1.98–2.56) × 1012 cm−2 eV−1 is located at ET in a range of (0.37–0.44) eV in the thin sample, while the trap state density of (2.3–2.92) × 1012 cm−2 eV−1 is located at ET in a range of (0.33–0.38) eV in the thick one. It indicates that the trap states in the thin sample are deeper than those in the thick one.

(Invited) The Stability of High Voltage AlGaN/GaN HEMTs

Si-based semiconductor devices have long dominated the power device market and are approaching their performance limit because of the inherent material limitations. On the other hand, wide bandgap semiconductors such as GaN and SiC promise significant performance boost coming from the superior material properties, such as a high critical electric field (3x106 V/cm), a high peak electron saturation velocity (2.5x107 cm/s), and a wide bandgap (3.4 eV) in GaN case [1]. In the III-nitride material system, a popular structure for power switching devices is the AlGaN/GaN HEMTs, because of the excellent two-dimensional electric gas (2DEG) in the hetrojunction interface, which greatly reduces the on-resistance of the device. Combined with the enhanced breakdown voltage from a large critical electric field, the figure of merit BV2/Ron,sp of reported AlGaN HEMTs have far surpassed the Si limit. The enhancement mode (E-mode) operation is strongly preferred for low power consumption, driver circuit simplicity, and fail-safe requirements for power electronic circuits. For AlGaN/GaN HEMTs realized utilizing epitaxial layers on the gallium face, the piezoelectric polarization, together with the spontaneous polarization, greatly enhances the 2DEG concentration, which is beneficial for reducing the on-resistance. But this makes it difficult to achieve the E-mode operation since the 2DEG under the gate region must vanish at zero gate bias. Several approaches have been proposed, including the recess gate structure, fluoride plasma treatment, the nano-ribbon channel, the p-GaN gate, and others. Although AlGaN/GaN HEMTs have demonstrated excellent performance with E-mode operation, their stability and reliability remains to be inspected before they can be commercialized. With different approaches to realize E-mode, the gate related stability problems arise differently depending on the gate design. In this work, we systematically investigate the stability of different device structures under different gate bias conditions, including Schottky gate HEMTs, p-GaN gate HEMTs, recessed MIS gate HEMTs, and multi-layer fluorinated MIS gate HEMTs. Figure 1 shows the pulsed Id-Vg characteristics of a recessed MIS gate AlGaN/GaN HEMT on a silicon substrate. The gate insulator is a 30 nm ALD deposited Al2O3 layer. The gate length is 3 μm and the gate-to-drain spacing is 20 μm for >1kV breakdown voltage. The pulse width is 5 ms and the period is 100 ms for all cases. The base voltage was varied from -15 V to 15 V and the drain current was measured during the pulse duration as shown in the inset. The threshold voltage Vth, which is defined when Id is 1mA/mm, increases with the base voltage, implying that negative charges are trapped in the gate insulator during the base duration. Figure 2 summarizes the extracted Vth for different gate structures using pulsed Id-Vg measurement with different gate base voltages. The Schottky gate HEMT shows a very stable Vth and Id-Vd characteristics against different pulse conditions but the negative Vth makes them less favorable. The p-GaN gate HEMT exhibits a positive but unstable Vth. The Vth shift can be as large as 1 V. Further annealing at 550oC in an O2ambient improves the p-type contact and hence the stability of the Vth but still limits the maximum gate voltage to 3 V. The decrease in Vth with increased gate base voltages implies trapping of holes in the p-GaN gate, which differs from the MIS gate device [2]. The MIS gate HEMT shows a positive threshold voltage about 6 V and a relatively large gate voltage swing. But the variation in the threshold voltage even at low gate base voltages indicates the occurrences of trapped charges in the gate insulator, which requires further examination and process improvement. More detailed discussion and measurement results will be shown. Acknowledgement This project was funded in part by financial support from Ministry of Economic Affairs 103A0002J9. Reference [1] T. P. Chow, R. Tyagi, IEEE Trans. Electron Devices 41(1994) 1481. [2] T.-F. Chang et al. to appear in IEEE Trans. Electron Devices. Figure 1

Charge balancing in GaN-based 2-D electron gas devices employing an additional 2-D hole gas and its influence on dynamic behaviour of GaN-based heterostructure field effect transistors

GaN-based heterostructure FETs (HFETs) featuring a 2-D electron gas (2DEG) can offer very attractive device performance for power-switching applications. This performance can be assessed by evaluation of the dynamic on-resistance Ron,dyn vs. the breakdown voltage Vbd. In literature, it has been shown that with a high Vbd, Ron,dyn is deteriorated. The impairment of Ron,dyn is mainly driven by electron injection into surface, barrier, and buffer traps. Electron injection itself depends on the electric field which typically peaks at the gate edge towards the drain. A concept suitable to circumvent this issue is the charge-balancing concept which employs a 2-D hole gas (2DHG) on top of the 2DEG allowing for the electric field peak to be suppressed. Furthermore, the 2DEG concentration in the active channel cannot decrease by a change of the surface potential. Hence, beside an improvement in breakdown voltage, also an improvement in dynamic behaviour can be expected. Whereas the first aspect has already been demonstrated, the second one has not been under investigation so far. Hence, in this report, the effect of charge-balancing is discussed and its impact on the dynamic characteristics of HFETs is evaluated. It will be shown that with appropriate device design, the dynamic behaviour of HFETs can be improved by inserting an additional 2DHG.

Reduction of contact resistance on AlGaN/GaN HEMT structures introducing uneven AlGaN layers

A new technique to reduce contact resistance on AlGaN/GaN HEMTs was proposed. This method introduced uneven AlGaN layer structures. 2D device simulations revealed a clear increase in two‐dimensional electron gas (2DEG) concentration near the edge regions where the AlGaN layer thickness fluctuated. This fringing effect is useful because metal and high‐density 2DEG can approach each other, overcoming an inherent tradeoff involving the AlGaN layer thickness. Experiments demonstrated the effectiveness of this technique, in which contact resistances obtained on some structures with uneven AlGaN layers were lower than the lowest limit for conventional flat AlGaN structures.

Growth and characterization of AlGaN/GaN/AlGaN double-heterojunction high-electron-mobility transistors on 100-mm Si(111) using ammonia-molecular beam epitaxy

To improve the confinement of two-dimensional electron gas (2DEG) in AlGaN/GaN high electron mobility transistor (HEMT) heterostructures, AlGaN/GaN/AlGaN double heterojunction HEMT (DH-HEMT) heterostructures were grown using ammonia-MBE on 100-mm Si substrate. Prior to the growth, single heterojunction HEMT (SH-HEMT) and DH-HEMT heterostructures were simulated using Poisson-Schrödinger equations. From simulations, an AlGaN buffer with “Al” mole fraction of 10% in the DH-HEMT was identified to result in both higher 2DEG concentration (∼1013 cm−2) and improved 2DEG confinement in the channel. Hence, this composition was considered for the growth of the buffer in the DH-HEMT heterostructure. Hall measurements showed a room temperature 2DEG mobility of 1510 cm2/V.s and a sheet carrier concentration (ns) of 0.97 × 1013 cm−2 for the DH-HEMT structure, while they are 1310 cm2/V.s and 1.09 × 1013 cm−2, respectively, for the SH-HEMT. Capacitance-voltage measurements confirmed the improvement in the confinement of 2DEG in the DH-HEMT heterostructure, which helped in the enhancement of its room temperature mobility. DH-HEMT showed 3 times higher buffer break-down voltage compared to SH-HEMT, while both devices showed almost similar drain current density. Small signal RF measurements on the DH-HEMT showed a unity current-gain cut-off frequency (fT) and maximum oscillation frequency (fmax) of 22 and 25 GHz, respectively. Thus, overall, DH-HEMT heterostructure was found to be advantageous due to its higher buffer break-down voltages compared to SH-HEMT heterostructure.

Experimental research on the new Al0.25Ga0.75N/GaN HEMTs with a step AlGaN layer

In this paper, experimental results are reported about the new Al0.25Ga0.75N/GaN high electron mobility transistor (HEMT) with a step AlGaN layer. The rule of 2DEG concentration variation with the thickness of AlGaN epitaxial layer has been applied to the new AlGaN/GaN HEMTs: The step AlGaN layer is formed at the gate edge by inductively coupled plasma etching, the 2DEG concentration in the etched region is much lower than the other parts of the device. A new electric field peak appears at the corner of the step AlGaN layer. The high electric field at the gate edge is decreased effectively due to the emergence of the new electric field peak, and this optimizes the surface electric field of the new AlGaN/GaN HEMTs. The new devices have the same threshold voltage and transconductance as the conventional structure, -1.5 V and 150 mS/mm. That means, the step AlGaN layer does not affect the forward characteristics of the AlGaN/GaN HEMTs. As the more uniform surface electric field distribution usually leads to a higher breakdown voltage (BV), with the same gate to drain length LGD=4 m, the BV can be improved by 58% for the proposed Al0.25Ga0.75N/GaN HEMTs as compared with the conventional structure. At VGS=1 V, the saturation currents (Isat) is 230 mA/mm for the conventional Al0.25Ga0.75N/GaN HEMT and 220 mA/mm for the partially etched Al0.25Ga0.75N/GaN HEMT (LEtch=4 m, LGD=4 m). The decrease of Isat is at most 10 mA/mm. However, as the BV has a significant enhancement of almost 40 V, these drawbacks are small enough to be acceptable. During the pulse I-V test, the current collapse quantity of the conventional structure is almost 40% of the maximum IDS(DC), but this quantity in the new devices is only about 10%, thus the current collapse effect in Al0.25Ga0.75N/GaN HEMTs has a significant remission for a step AlGaN layer. And as the high electric field peak at the gate edge is decreased, the effect of the gate electrode on electron injection caused by this electric field peak is also included. The injected electrons may increase the leakage current during the off-state, and these injected electrons would form the surface trapped charge as to decrease the 2DEG density at the gate. As a result, the output current and the transconductance would decrease due to the decreased electron density during the on-state. That means, with the region partially etched, the electron injection effect of the gate electrode would be remissed and the stability of Schottky gate electrode would be improved. In addition, due to the decrease of the high electric field at the gate edge, the degradation of the device, which is caused by the high electric field converse piezoelectric effect, will be restrained. The stability of the partially etched AlGaN/GaN HEMT will become better.

Influence of strain induced by AlN nucleation layer on the electrical properties of AlGaN/GaN heterostructures on Si(111) substrate

The crack-free metal-organic chemical vapor deposition (MOCVD) grown AlGaN/GaN heterostructures on Si substrate with modified growth conditions of AlN nucleation layer (NL) and its influence on the electrical and structural properties of conductive GaN layer are presented. From the Hall electrical measurements, a gradual decrease of two-dimensional electron gas (2DEG) concentration near heterointerface as the function of NL thickness is observed possibly due to the reduction in difference of piezoelectric polarization charge densities between AlGaN and GaN layers. It also indicates that the minimum tensile stress and a relatively less total dislocation density for high pressure grown NL can ensure a 20 % increment in mobility at room temperature irrespective of the interface roughness. The thickness and pressure variations in NL and the subsequent changes in growth mode of AlN contributing to the post growth residual tensile stress are investigated using X-ray diffraction and Raman scattering experiments, respectively. The post growth intrinsic residual stress in top layers of heterostructures arises from lattice mismatches, NL parameters and defect densities in GaN. Hence, efforts to reduce the intrinsic residual stress in current conducting GaN layer give an opportunity to further improve the electrical characteristics of AlGaN/GaN device structures on Si.

Gate metal dependent electrical characteristics of AlGaN/GaN HEMTs

We investigated transfer characteristics of AlGaN/GaN high electron mobility transistors (HEMTs) and the effect of the barrier height of Schottky gate metals. It is found that the threshold voltage of the HEMT structures with the Ni Schottky contact shows a positive shift compared to that of the Ti Schottky contacts (ΔVth=2.9V). The maximum saturation current of the HEMT structures with the Ti Schottky contact (∼1.4×107A/cm2) is found to be ∼2.5 times higher than that of the Ni Schottky contact (2.9×107A/cm2). The conduction mechanisms have been examined by comparing the experimental results with numerical simulations, which confirm that the increased barrier height is mainly attributed to the reduction of 2-DEG concentration.

(Invited) Growth and Characterization of High Power AlInN/GaN HEMTs

AlInN, a material lattice-matched to GaN, is very promising for high current, high transconductance, and high RF performance high-electron-mobility transistors (HEMTs). However, it is challenging to grow high quality AlInN HEMTs due to the large immiscibility and distinct thermal stability of AlN and InN, which causes premature voltage breakdown. Several methods have been proposed to solve this problem, such as using a thin AlInN barrier layer and field plate structure (VB = 400 V), Schottky source/drain (VB = 460 V), or 5 % AlGaN back barrier layer (VB = 3 KV). However, most of these methods are still problematic in maintaining a low Ron and high VB simultaneously. GaN cap has been widely used to achieve good surface smoothness and reduce current collapse in AlGaN/GaN-based HEMTs. GaN MOS-HEMTs with a thick GaN cap layer to reduce the surface field for high breakdown voltage have also been reported (VB = 840 V). Our study shows that GaN cap layer also has a positive influence on AlInN HEMTs in reducing current collapse phenomenon, dissipating electric field near the gate edge and enhancing the breakdown voltage. By using a GaN cap layer, AlInN-based HEMTs, exhibiting low Ron and high VB simultaneously, have been demonstrated on 6 inch silicon substrates in this work.Two AlInN-HEMT structures denoted as sample A and B were prepared by metal-organic chemical vapor deposition on 6 inch Si (111) substrates. Sample A is a typical HEMT structure without a GaN cap, while sample B has an additional 13 nm-thick GaN cap layer on top of the AlInN layer. Room temperature Hall mobility, two-dimensional electron gas (2DEG) concentration and sheet resistance (Rsh) of sample A are 886 cm2/V.s, 2.49x1013 cm-2 and 283 ohm/sq, while those of sample B are 1,450 cm2/V.s, 1.61x1013 cm-2 and 267 ohm/sq, respectively. Due to the reverse polarization effect of GaN cap on sample B, the 2DEG concentration of sample B is lower than that of sample A.Dynamic Ron of power electronics reflects acceptor-like defect in the material which could be improved by decreasing electric field at the gate edge or decreasing the defect density in the material. Devices were tested under off-state condition with a constant Vd for 1 second, then switched to on-state to observe the variation of Ron. Sample A exhibits serious degradation on dynamic Ron with time. The ratio of dynamic Ron over static Ron of sample A reaches 5 under 20 V of Vd stress, while that of sample B can be maintained at 1.3 under the same test conditions. As the off-state stress Vd increases, the dynamic Ron ratio of sample B also increases significantly and reaches 5 under 60 V of Vd stress. The improvement of dynamic Ron on AlInN HEMT with a GaN cap implies the trap density of sample B is significantly reducedThe on-state I-V characteristics of the HEMTs shows that Id,max of sample A and B at Vg of 0 V are 480 mA/mm and 660 mA/mm, respectively. Thus, the extracted Ron is improved from 4.6 mohm-cm2 of sample A to 3.7 mohm-cm2 of sample B. This is consistent with their Hall results. The extrapolated threshold voltage (Vth ) of sample B is -9.1 V, which is larger than -6.3 V of sample A due to its thicker intermediate layer and the presence of the GaN cap layer. With the help of a GaN cap layer, which disperses the electrical field near the gate edge, sample B has a lower drain leakage current than sample A. While the VB of sample A reaches 530 V, indicating the crystal quality of AlInN is comparable to that of the traditional AlGaN-based HEMTs, the GaN cap extends the breakdown voltage of sample B to 675 V.

Breakdown voltage analysis for the new Al0.25 Ga0.75N/GaN HEMTs with the step AlGaN layers

In order to optimize the surface electric field of AlGaN/GaN high electron mobility transistors (HEMTs), a novel AlGaN/GaN HEMT has been grown with a step AlGaN layer, made for the first time as far as we know, to improve the breakdown voltage. The discipline of the 2DEG concentration varying with the thickness of the AlGaN epitaxy layer has been applied to the new AlGaN/GaN HEMTs with AlGaN/GaN heterostructure. By thinning the AlGaN layer near the gate edge, the 2DEG concentration in the channel is made to form the low concentration region near the gate edge. New electric field peak has appeared at the corner of the step AlGaN layer. The high electric field has been decreased effectively due to the emergence of new electric field peak; this optimizes the surface electric field of the new AlGaN/GaN HEMTs. Then the breakdown voltage is improved to 640 V in the new AlGaN/GaN HEMTs with the step AlGaN layer as compared with 446 V for the conventional structure. In order to let the breakdown curve consistent with the test results, a certain concentration of the acceptor-like traps is added to the GaN buffer to capture the leaking current coming from the source electrode. Simulation results verify the causes for doping acceptor type ions to the GaN buffer, given by foreign researchers. The breakdown curves have been obtained which are consistent with the test results in this paper.

AlN隔离层生长时间对AlGaN/AlN/GaN HEMT材料电学性能的影响

利用金属有机化学气相沉积(MOCVD)设备,在蓝宝石(0001)面上外延不同生长时间AlN隔离层的AlxGa1-xN/AlN/GaN结构的高电子迁移率的晶体管(HEMT),研究了AlN隔离层厚度对HEMT材料电学性能的影响。研究发现采用脉冲法外延(PALE)技术生长AlN隔离层的时间为12 s(1 nm左右)时,HEMT材料的方块电阻最小,电子迁移率为1 500 cm2·V-1·s-1,二维电子气(2DEG)浓度为1.16×1013cm-2。AFM测试结果表明,一定厚度范围内的AlN隔离层并不会对材料的表面形貌产生重大的影响。HRXRD测试结果表明,AlGaN/AlN/GaN具有好的异质结界面。

High sensitivity Hall devices with AlSb/InAs quantum well structures

AlSb/InAs quantum well (QW) structures and InAs films on GaAs (001) substrates were grown by molecular beam epitaxy (MBE). We investigated the dependence of electron mobility and two-dimensional electron gas (2DEG) concentration on the thickness of an InAs channel. It is found that electron mobility as high as 19050 cm2 · V−1 · s−1 has been achieved for an InAs channel of 22.5 nm. The Hall devices with high sensitivity and good temperature stability were fabricated based on the AlSb/InAs QW structures. Their sensitivity is markedly superior to Hall devices of InAs films.

Influence of growth process scheme on the properties of AlGaN/AlN/GaN heterostructures

AbstractTo check the influence of different process parameters, such as precursors (TMAl and TMGa) flow rates, growth interrupt time between subsequent stages of the growth for purging the reactor, on the properties of the two dimensional electron gas (2DEG) in the AlGaN/AlN/GaN heterostructure, seven sets of HEMT structures were developed and investigated. It was demonstrated that for the growth of canonically the same “sandwich” structure, several growth process schemes can be applied, and different properties of fabricated AlGaN/AlN/GaN can be obtained. It was shown, that changing Al content in the AlGaN barrier layer by modifying TMAl flow rate results in significant decrease in 2DEG mobility compared to the method in which TMGa flow rate is adjusted to keep the assumed composition. The presented results also showed, that the growth interruption between successive stages of AlGaN/AlN/GaN growth has a distinct impact on the mobility and concentration of 2DEG in HEMT’s. The transport properties of the fabricated heterostructures were studied by impedance spectroscopy technique. Specific parameters of growth process, such as growth rate and Al content, were obtained by in‐situ interferometry and photoluminescence technique. There are many publications which describes the influence of different structure parameters, e.g. composition of the AlGaN barrier layer, thickness of the AlN interlayer, however the information is rather fragmentary and it is difficult to compare quantitative results deriving from various groups. The purpose of this article was to show the key HEMT’s structure parameters in relation to MOVPE process scheme, influencing the electrical transport of 2DEG. Detailed explanation of physical phenomenas was not the main aim of this work and will be presented later. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Lateral modulation doping of two‐dimensional electron or hole gas

AbstractWe show that the two‐dimensional electron gas (2DEG) injected from the region with a higher concentration into the region with a lower concentration decays very slowly. In contrast to the three‐dimensional case, where the concentration is an exponential function of distance from the injecting contact, the 2D concentration is inversely proportional to this distance. Moreover, using passivation with a high dielectric constant or a floating metal gate placed close to the 2DEG (with separation smaller than the Bohr radius) could dramatically enhance the 2DEG concentration decay length. This effect enables lateral remote electron transfer in 2DEG, similar to remote doping in heterostructures, and can be used to optimize the electric field and/or electron concentration profile.