Thin AlGaN Layer Research Articles

Impact of selective thermal etching in mixed H2/NH3 atmosphere on crystal quality of AlGaN/GaN heterostructures

We investigated the impact of selective thermal etching in a mixed hydrogen and ammonia atmosphere on the crystal quality and electrical characteristics of Ga- and N-polar AlGaN/GaN heterostructures. It was revealed that the etching rate of N-polar GaN is lower than that of Ga-polar GaN under our experimental conditions, and they showed a similar dependence on process temperature with almost the same activation energies. We demonstrated the use of a thin AlGaN layer as a selective etching stopper for both Ga- and N-polarity. The AlGaN stoppers exhibited a smooth surface after etching the GaN layer above them. As for the electrical characteristics, there was no significant degradation in the mobility of the two-dimensional electron gas. The results indicate that selective thermal etching is a promising technique for device fabrication and is especially suitable for precise GaN layer removal when GaN-based devices are fabricated with an epitaxial layer transfer technique.

Smooth, low rate, selective GaN/AlGaN etch

The aluminum content is widely used in III-N semiconductors as a determiner of material etch characters. Applications consisting of thin GaN/AlGaN heterostructures can afford a maximum of 6 nm of a thin AlGaN layer over etch demand complex processes with precise low etch rates and low etched GaN surface roughness. In this paper, the effects of bias power and SF6 flow ratios on the chlorine chemistry etch rate, selectivity, and GaN surface roughness are investigated in a high-frequency bias generator and low power inductively coupled plasma configuration. Bias power and SF6 flow control the etch responses and are used to find the optimal spots for low etch rate, low GaN roughness, and high GaN:AlGaN selectivity for the fabrication of devices consisting of thin GaN/AlGaN heterostructures. The results are compared with the other selective gas chemistries and the more common 13.56 MHz frequency bias generator.

Radiation-Induced DC Parametric Degradation of Enhancement Mode GaN-on-(111)Silicon High-Electron-Mobility Transistors

AlGaN/GaN high-electron-mobility transistors (HEMTs) for applications in radiation environments has required the exact determination of their radiation hardness. Although bulk GaN is the preferred substrate for the epitaxial growth of GaN and AlGaN, (111) silicon is a possible substrate for HEMT devices. The stability of electrical parameters of enhancement-mode (E-mode) GaN HEMTs under gamma irradiation is an essential characterization for reliability. The present work reports the transfer characteristics degradation of E-mode GaN HEMTs under various total dose irradiation from 5krad up to 60Mrads. This represents the first comprehensive total dose study ranging from a few krads up the tens of Mrads. The measured device-to-device variations within groups of devices are presented. The evolutions of critical DC parameters with accumulated total dose is found to be significant at low doses and saturates at high dose levels. The degradation mechanism is very similar to the reported radiation-induced shallow electron traps that trap and release electrons in the near surface GaN and AlGaN layers. Due to the unique properties including high breakdown voltage, high electron mobility and high thermal stability, wide-bandgap semiconductor power electronic devices, especially Gallium Nitride (GaN) high-electron-mobility transistors (HEMTs), have shown great promise in space, defense and nuclear applications, which often involve radiation-hardened electronics that require high radiation tolerance to fluxes of neutrons or gamma ray of devices. To turn the promising potential of GaN HEMTs into reality, investigations on the influence of irradiation on the device performance and reliability are essential. Many studies have investigated the radiation hardness of group III–nitride compound semiconductor devices under various irradiation types, such as electrons, protons, neutrons, and X-rays and at various total irradiation doses. They generally showed higher radiation tolerance compared with Si because of high displacement energy of GaN, demonstrating the favorability of GaN devices in hard radiation environments. Many studies have investigated the influence of 60Co gamma radiation on the device electrical characteristics. However, they were all focusing on depletion mode (D-mode) devices. Enhancement mode (E-mode) GaN-on-Si transistors have been commercially available for years. Their major advantages over D-mode devices are elimination of negative voltage supply and the enabling of direct-coupled FET logic (DCFL) for digital circuits. They also improve the efficiency of commercial DC-DC converters in various power levels and topologies. A previous work on gamma radiation of E-mode GaN-on-Si HEMTs only reported cumulative total doses of up to 1 Mrads (Si). The present work provides a complete coverage of low-to-high irradiation doses from 5krads up to 60Mrads (Si), for commercially available E-mode GaN HEMTs and the devices are characterized to gauge variations in the critical DC parameters. To the best of the author’s knowledge, the present work is the first to investigate the gamma radiation tolerance of E-mode GaN HEMTs covering such a wide range of total doses. This facilitates the observation of the complete degradation behavior of DC parameters in hard radiation applications of the E-mode GaN HEMTs. The devices were supplied in the form of 0.9mm square passivated die with round solder bumps for contacts. They have a rated continuous drain-to-source voltage VDS of 60V, and a rated continuous drain current ID of 1A. These GaN-on-Si devices were fabricated on Si wafers considering process compatibility and cost. An aluminum nitride (AlN) thin layer was grown on the Si wafer to be a seed layer for the AlGaN/GaN heterostructure. Subsequently, the AlGaN/GaN heterostructure was grown on the AlN layer, with a thin AlGaN layer grown above the highly resistive GaN serving as a strained interface between the GaN and AlGaN crystal layers, allowing the creation of a two-dimensional electron gas (2DEG) filled with abundant free electrons with ultra-high mobility. The gate electrode was processed subsequently forming a depletion region under the gate. As an E-mode transistor, the device is turned on by applying a positive bias to the gate. All the irradiations were conducted at room temperature by 60Co gamma-rays with a flux of 318.5Rads (Si)/s, at the dry-cell panoramic gamma irradiator in the University of Maryland Radiation Facilities. The entire irradiation procedure consists of two parts. The first part is higher-dose irradiation with total doses of up to 60Mrads based on two stages. At the first stage, the devices were irradiated at 10Mrads, while at the second stage, the devices were irradiated for another 50Mrads, with three of them taken out after 25Mrads during the second stage. The second part is lower-dose irradiations with total doses of up to 2Mrads. The temperature throughout the irradiation process was monitored to be below 30°C.

Polarization Engineering to Manipulate the Breakdown Voltage for GaN‐Based PIN Diodes

This study proposes to increase the breakdown voltage for GaN‐based PIN diodes using the polarization effect, such that a thin AlGaN layer is inserted into the drift layer to modulate the electric field profiles, and by properly designing the device architecture, the electric field in the drift layer can be remarkably reduced by the polarization effect, and this enables the enhancement of the breakdown voltage. The study further investigates the parametric sensitivity of the breakdown voltage for the proposed GaN‐based PIN diodes to different device structures with various drift layer thicknesses. Moreover, it is found that the position of the inserted AlGaN thin layer is also important in affecting the breakdown voltage, such that the AlGaN insertion layer reduces the electric field with the maximum intensity in the drift layer.

Precise determination of polarization fields in c-plane GaN/AlxGa1-xN/GaN heterostructures with capacitance–voltage-measurements

Due to changes in the spontaneous and piezoelectric polarization, AlGaN/GaN heterostructures exhibit strong polarization fields at heterointerfaces. For quantum wells, the polarization fields lead to a strong band bending and a redshift of the emission wavelength, known as quantum-confined Stark effect. In this paper the polarization fields of thin AlGaN layers in a GaN matrix were determined by evaluating the changes in the depletion region width in comparison to a reference sample without heterostructure using capacitance–voltage-measurements. The polarization fields for Al0.09Ga0.91N (0.6 ± 0.7 MV cm−1), Al0.26Ga0.74N (2.3 ± 0.6 MV cm−1), Al0.34Ga0.66N (3.1 ± 0.6 MV cm−1), Al0.41Ga0.59N (4.0 ± 0.7 MV cm−1) and Al0.47Ga0.53N (5.0 ± 0.8 MV cm−1) heterostructures were determined. The results of the field strength and field direction of all samples are in excellent agreement with values predicted by theory and a capacitance–voltage based Poisson-carrier transport simulation approach giving experimental evidence for a nonlinear increasing polarization field with Al-concentration.

AlGaN nano-shell structure on a GaN nanorod formed with the pulsed MOCVD growth

An AlGaN/GaN multi-shell structure on a GaN nanorod (NR) is formed by using the self-catalytic pulsed growth process of metalorganic chemical vapor deposition with Ga and Al/N supplies in the first and second half-cycles, respectively. With Al supply, a thin AlGaN layer is precipitated near the end of a growth cycle to form the AlGaN/GaN structure. Because of the lower chemical potential for GaN nucleation, when compared with AlN, a GaN layer is first deposited in a growth cycle. AlGaN is not precipitated until the AlN nucleation probability becomes higher when the catalytic Ga droplet is almost exhausted. Because the Al adatoms on the NR sidewalls hinder the upward migration of Ga adatoms for contributing to the Ga droplet at the NR top, the size of the Ga droplet decreases along growth cycle leading to the decrease of GaN layer thickness at the top until a steady state is reached. In this process, the slant facet of an NR changes from the (1–102)-plane into (1–101)-plane. To interpret the observed growth behaviors, formulations are derived for theoretically modeling the AlN nucleation probability, NR height increment in each growth cycle, and the time of exhausting the Ga droplet in a cycle.

Design of InGaN-ZnSnN2 quantum wells for high-efficiency amber light emitting diodes

InGaN-ZnSnN2 based quantum wells (QWs) structure is proposed and studied as an active region for high efficiency amber (λ ∼ 600 nm) light emitting diodes (LEDs), which remains a great challenge in pure InGaN based LEDs. In the proposed InGaN-ZnSnN2 QW heterostructure, the thin ZnSnN2 layer serves as a confinement layer for the hole wavefunction utilizing the large band offset at the InGaN-ZnSnN2 interface in the valence band. The barrier layer is composed of GaN or AlGaN/GaN in which the thin AlGaN layer is used for a better confinement of the electron wavefunction in the conduction band. Utilizing the properties of band offsets between ZnSnN2 and InGaN, the design of InGaN-ZnSnN2 QW allows us to use much lower In-content (∼10%) to reach peak emission wavelength at 600 nm, which is unachievable in conventional InGaN QW LEDs. Furthermore, the electron-hole wavefunction overlap (Γe-h) for the InGaN-ZnSnN2 QW design is significantly increased to 60% vs. 8% from that of the conventional InGaN QW emitting at the same wavelength. The tremendous enhancement in electron-hole wavefunction overlap results in ∼225× increase in the spontaneous emission radiative recombination rate of the proposed QW as compared to that of the conventional one using much higher In-content. The InGaN-ZnSnN2 QW structure design provides a promising route to achieve high efficiency amber LEDs.

New Architecture of ZnGeN2/In0.16Ga0.84N Type‐II Quantum Well‐Based Green Emitting LED

We report here on a new green‐emitting light emitting device (LED) architecture containing only 16% of indium. The structure is based on the use of a new type‐II ZnGeN2/In0.16Ga0.84N quantum well. The zinc germanium nitride (ZnGeN2) is a new promising semiconductor for optoelectronic devices such as LEDs or photovoltaic cells due to its large, direct, and adjustable band gap, most particularly considered to overcome the green‐gap issue in LED technology. Studies of ZnGeN2‐based quantum well behaviors are scarce and no information on the overall electro‐optical operation of such LED is available. We simulate here the complete behavior of a green LED structure in which the active region is a type‐II ZnGeN2/In0.16Ga0.84N quantum well. A thin AlGaN layer is used as a barrier for a better carrier confinement. The position and the thickness of the ZnGeN2 layer are parameters used to examine the luminous and electrical behavior as well as the external quantum efficiency of this LED compared to a standard In0.22Ga0.78N LED emitting at the same wavelength. We demonstrate a significant enhancement of the spontaneous emission and the possibility to reach both a better quantum efficiency and light output when using the type‐II structure.

Efficient III-Nitride MIS-HEMT devices with high-κ gate dielectric for high-power switching boost converter circuits

The paper reports the results of a systematic theoretical study on efficient recessed-gate, double-heterostructure, and normally-OFF metal-insulator-semiconductor high-electron mobility transistors (MIS-HEMTs), HfAlOx/AlGaN on Al2O3 substrate. In device architecture, a thin AlGaN layer is used in the AlGaN graded barrier MIS-HEMTs that offers an excellent enhancement-mode device operation with threshold voltage higher than 5.3 V and drain current above 0.64 A/mm along with high on-current/off-current ratio over 107 and subthreshold slope less than 73 mV/dec. In addition, a high OFF-state breakdown voltage of 1200 V is achieved for a device with a gate-to-drain distance and field-plate length of 15 μm and 5.3 μm, respectively at a drain current of 1 mA/mm with a zero gate bias, and the substrate grounded. The numerical device simulation results show that in comparison to a conventional AlGaN/GaN MIS-HEMT of similar design, a graded barrier MIS-HEMT device exhibits a better interface property, remarkable suppression of leakage current, and a significant improvement of breakdown voltage for HfAlOx gate dielectric. Finally, the benefit of HfAlOx graded-barrier AlGaN MIS-HEMTs based switching devices is evaluated on an ultra-low-loss converter circuit.

Designs of blue and green light-emitting diodes based on type-II InGaN-ZnGeN2 quantum wells

Type-II InGaN-ZnGeN2 quantum wells (QWs) are studied as improved active regions for light-emitting diodes emitting in the blue (λ ∼ 485 nm) and green (λ ∼ 530 nm) spectral ranges. Both the energy band gap and the lattice parameters of ZnGeN2 are very close to those of GaN. The recently predicted large band offset between GaN and ZnGeN2 allows the formation of a type-II InGaN-ZnGeN2 heterostructure. The strong confinement of holes in the ZnGeN2 layer allows the use of a lower In-content InGaN QW to extend the emission wavelength into the blue and green wavelength regions, as compared to the traditional InGaN QW with uniform In content. In the type-II InGaN-ZnGeN2 QW designs, a thin AlGaN layer was used as a barrier for better carrier confinement. The type-II InGaN-ZnGeN2 QWs lead to a significant enhancement of the electron-hole wave function overlap as compared to those of the conventional QWs. Simulation studies of the proposed type-II QWs promise a significant enhancement of the spontaneous emission rate by 6.1–7.2 times for the QW design emitting at the blue wavelength region and 4.6–4.9 times for the QW design emitting at the green wavelength region, as compared to the conventional InGaN QWs emitting at the same wavelengths.

Simulation and Experimental Study on Barrier Thickness of Superlattice Electron Blocking Layer in Near-Ultraviolet Light-Emitting Diodes

The optical performance and relevant physical properties of near-ultraviolet (NUV) GaN-based light-emitting diodes (LEDs) are investigated. Specifically, the influence of traditional AlGaN bulk electron blocking layer (EBL) and AlGaN/GaN superlattice (SL) EBL with various thicknesses of AlGaN layers on NUV LEDs is explored. It is indicated from the band diagrams, electrostatic field profile, electron reflecting and hole transmitting spectra, and carrier concentrations profile that the use of a thin AlGaN layer of AlGaN/GaN SL EBL is beneficial to the electron confinement and hole injection in the active region, which results in the high internal quantum efficiency and low efficiency droop at high injection current. Moreover, the experimental results show that replacing the traditional AlGaN bulk EBL with the AlGaN/GaN SL EBL can markedly improve the optical performance. When compared with the NUV LED with traditional AlGaN bulk EBL, the output power of the NUV LED with the proposed AlGaN/GaN SL EBL increases from 13.5 to 48.7 mW at 100 mA.

Effect of Capping on Electrical and Optical Properties of GaN Layers Grown by HVPE

Gallium nitride, grown by hydride vapor phase epitaxy and capped with a thin AlGaN layer, was studied by photoluminescence (PL) methods. The concentration of free electrons in GaN was found from the time-resolved PL data, and the concentrations of point defects were estimated from the steady-state PL measurements. The intensity of PL from GaN decreases moderately after capping it with Si-doped AlGaN, and it decreases dramatically after capping with Mg-doped AlGaN. At the same time, the concentration of free electrons and the concentrations of main radiative defects in GaN are not affected by the AlGaN capping. We demonstrate that PL is a powerful tool for nondestructive characterization of semiconductor layers buried under overlying device structures.

Vertical Buffer Leakage and Temperature Effects on the Breakdown Performance of GaN/AlGaN HEMTs on Si Substrate

GaN-based lateral high electron mobility transistors (HEMTs) are highly attractive for high-power-switching applications, because they are foreseen to contribute significantly towards improving the efficiency and miniaturizing the power supplies, due to the low on-state resistance (RON) for a given blocking breakdown voltage (Vbr) and the low gate charge required for fast switching. Transistors with high lateral Vbr and low RON have been demonstrated by several groups. Most of the excellent results achieved for the lateral breakdown voltage (three terminal off-state breakdown) are reported for the substrate being in floating mode. As soon as the lateral Vbr measurement is performed within a substrate in grounded mode, the HEMT’s lateral Vbr degrades and affects the device performance. It is due to the vertical isolation of the buffer (two terminal off-state breakdown) through the leakage current which affects and dominates the lateral Vbr. The HEMTs breakdown voltages are limited by GaN-to-Si vertical leakage. Moreover, in practice the transistor back side needs to be grounded. Alike, little works are reported concerning the temperature effect on the HEMT’s lateral Vbr and the vertical buffer isolation, which is a reliability requirement for components operating at high temperature, as in automotive applications. To improve the lateral Vbr, thicker epitaxial buffer layers are used, but it causes a severe strain, increasing the wafer’s bow and leads to cracks formation. An alternative would be to optimize the carbon doping in the buffer layers, in combination with an appropriate thickness of the buffer’s layers. In this talk, we will present our recent results reporting to the electrical characterization of the 600 V designed GaN/AlGaN HEMTs, grown on 4-inch silicon substrate, including the breakdown voltage measurements of the lateral buffer isolation (substrate grounded) and the vertical buffer isolation. We compare the HEMTs off-state Vbr for substrate grounded or in floating mode. The HEMT breakdown voltage does not degrade up to 600 V bias (grounding substrate). We have also studied the effect of the substrate’s heating (up to 150°C) on the vertical buffer isolation characteristics, and on the Vbr performance of the HEMTs. The AlGaN/GaN HEMT heterostructure is grown on (111)-oriented p-type Si substrates by metal–organic chemical vapor deposition, with a total thickness of 5.6 µm. The epitaxial structure consists of a thin AlN nucleation layer, a thin AlGaN layer, followed by a carbon doped AlN/GaN superlattice and a carbon doped GaN buffer. Afterwards, we grow a 400 nm thick undoped GaN channel layer and a 20 nm unintentionally doped Al0.18Ga0.82N barrier layer. The normally-on HEMTs were fabricated using our standard technology, implantation for the isolation, annealed Ti/Al/Ni/Au stack as ohmic contacts, Ni/Au as Schottky gate metallization, and SiNx as a passivation layer. HEMTs with a gate length of 2 μm, gate–drain spacing of 15 μm, and gate width of 10 mm; 100 × 100 μm2 isolated contacts to characterize the vertical top-to-substrate leakage and two metallic lines with 15 µm distance for the lateral buffer isolation were fabricated. Figure 1 exhibits the mapping measurement comparison, across the 4-inch wafer, between the different blocking voltages of the buffer. As shown, the lateral buffer isolation (substrate floating) reaches 1700 V (blue curves), at an absolute current of 0.1 µA. The lateral blocking voltage decreases to 830 V (substrate grounded), due to the contribution of the vertical leakage current (red curves). The vertical two-terminal isolation blocking voltage (green curves) is also 830 V (substrate grounded), at a corresponding absolute leakage current of 0.1 µA or a current density of 10 µA/mm2. It confirms the domination of the vertical leakage current over the lateral leakage current, and the necessity to grow a buffer with a high vertical isolation to avoid the degradation of the HEMT’s breakdown voltage (grounded mode). The 10 mm gate periphery HEMTs achieve a (Vbr) of 600 V, at gate-source voltage of – 3 V, with corresponding IDS currents between 0.3 and 0.55 µA/mm (floating substrate), as depicted in the mapping measurements of Figure 2 (blue curves). When the substrate is grounded, no current degradation is observed and the HEMTs achieve again a Vbr of 600 V, with corresponding IDS currents between 0.2 and 0.4 µA/mm (red curves). The extracted vertical bulk current (green curves) is too low at 600 V bias, and does not deteriorate the lateral leakage current (grounded mode). This demonstrates the quality of our epitaxial structure. The influence of the substrate temperature, up to 150°C, on the buffer’s isolation voltage/current and HEMT’s Vbr will be discussed. Figure 1

Determination of Temperature-Dependent Stress State in Thin AlGaN Layer of AlGaN/GaN HEMT Heterostructures by Near-Resonant Raman Scattering

The temperature-dependent stress state in the AlGaN barrier layer of AlGaN/GaN heterostructure grown on sapphire substrate was investigated by ultraviolet (UV) near-resonant Raman scattering. Strong scattering peak resulting from the A1(LO) phonon mode of AlGaN is observed under near-resonance condition, which allows for the accurate measurement of Raman shifts with temperature. The temperature-dependent stress in the AlGaN layer determined by the resonance Raman spectra is consistent with the theoretical calculation result, taking lattice mismatch and thermal mismatch into account together. This good agreement indicates that the UV near-resonant Raman scattering can be a direct and effective method to characterize the stress state in thin AlGaN barrier layer of AlGaN/GaN HEMT heterostructures.

Capping green emitting (Ga,In)N quantum wells with (Al,Ga)N: impact on structural and optical properties

The difference of growth temperatures between InGaN quantum wells and GaN barriers has detrimental effects on the properties of the wells. Different capping processes of InGaN quantum well with a thin AlGaN layer have been investigated to prevent these effects. Both structural and optical properties of the samples, grown on c-plane sapphire substrates by metalorganic vapor phase epitaxy, were studied through transmission electron microscopy (TEM), x-ray diffraction and room temperature photoluminescence. The average quantum well thickness and its indium composition were determined by digital processing of lattice fringes in cross-sectional TEM images. From the analysis of the well thickness distribution, it is shown that AlGaN as a capping layer helps to compensate an unwanted undulation at the upper InGaN QW-barrier interface. Moreover, when deposited at the same temperature as InGaN, the AlGaN layer is effective in avoiding or reducing the evaporation and/or diffusion of indium from InGaN wells, which results in the thinning of the well. It therefore helps to extend the emission wavelength up to 540 nm with a reduced degradation of the room temperature photoluminescence efficiency.

High-Temperature Characteristics in Recessed-Gate AlGaN/GaN Enhancement-Mode Heterostructure Field Effect Transistors with Enhanced-Barrier Structures

Recessed-gate AlGaN/GaN enhancement-mode (E-mode) heterostructure field-effect transistors (HFETs) with enhanced-barrier structures were fabricated, and their high-temperature characteristics were examined. Owing to enhanced-barrier structures, where a thin AlGaN layer with a higher Al composition is inserted into the AlGaN barrier, excellent E-mode operation with threshold voltage (Vth) higher than +3 V and drain current density (Id) higher than 600 mA/mm was obtained at room temperature. Interestingly, Id did not decrease much at high temperatures, i.e., Id decreased from 610 to 590 mA upon a temperature raise from RT to 300 °C. Open-gate devices with and without recessed-gate structures were fabricated and their two-terminal characteristics were comparatively examined, revealing that the observed peculiar characteristics are ascribed to recessed-gate structures. A model for explaining the observed high-temperature characteristics is proposed.

Comparison of Semiconductor–Electrolyte and Semiconductor–Metal Schottky Junctions Using AlGaN/GaN Photoelectrochemical Electrode

A semiconductor photoelectrochemical electrode that contains a heterostructure at the surface vicinity is attractive as a corrosion-tolerant electrode. In order to clarify its basic characteristics, the differences in Schottky junctions were evaluated using the semiconductor capacitance dependence on voltage. The Mott–Schottky relationship of a thin AlGaN layer on GaN was different from that of GaN bulk. The Schottky junctions were formed not only by using an electrolyte contact, but also by using a metal electrode to evaluate the effects of the semiconductor–electrolyte interface. Although diffusions of ions and solvents occur in the electrolyte, the Mott–Schottky plot measured for the electrolyte system showed a similar voltage dependence slope to that measured with a metal contact. This indicates that an electrolyte has a limited effect on the depletion behavior of the semiconductor heterojunction electrode.

Origin of InGaN light-emitting diode efficiency improvements using chirped AlGaN multi-quantum barriers

We analyze efficiency droop reductions in InGaN/GaN light-emitting diodes caused by a chirped AlGaN/GaN multi-quantum barrier (MQB). Such electron barriers are expected to create an additional forbidden energy range above the natural conduction band edge, which reduces the electron leakage current. Advanced numerical device simulations reveal that energy band bending practically eliminates this MQB effect. Instead, we find that the measured efficiency improvement has its origin in enhanced hole injection, which can be more easily accomplished using a single thin AlGaN layer.

Contactless electroreflectance study of band bending for undoped, Si- and Mg-doped GaN layers and AlGaN/GaN transistor heterostructures

The surface band bending in undoped, Si-doped and Mg-doped GaN layers with Ga-face polarity as well as AlGaN/GaN heterostructures with Ga(Al)-face polarity has been investigated by room temperature contactless electroreflectance (CER) spectroscopy. The opposite phase of CER resonance (i.e., opposite band bendings) has been observed for n-type (undoped and Si-doped) and p-type (Mg-doped) GaN layers. It means that for thick GaN layers the surface band bending results not from crystal polarity but from the Fermi-level pinning at the surface and carrier type/concentration inside the layer. The crystal polarity can influence the surface band bending for thin (Al)GaN layers for which the screening phenomena can be neglected or are very weak. Such a situation is typical of AlGaN/GaN transistor structures where the thickness of AlGaN layer is below 40nm. In this case, the strong internal electric field in AlGaN layer is manifested in CER spectra by a resonance with a long period Franz–Keldysh oscillation.

Effects of interface states and temperature on the C-V behavior of metal/insulator/AlGaN/GaN heterostructure capacitors

The impact of states at the insulator/AlGaN interface on the capacitance-voltage (C-V) characteristics of a metal/insulator/AlGaN/GaN heterostructure (MISH) capacitor was examined using a numerical solver of a Poisson equation and taking into account the electron emission rate from the interface states. A parallel shift of the theoretical C-V curves, instead of the typical change in their slope, was found for a MISH device with a 25-nm-thick AlGaN layer when the SiNx/AlGaN interface state density Dit(E) was increased. We attribute this behavior to the position of the Fermi level at the SiNx/AlGaN interface below the AlGaN valence band maximum when the gate bias is near the threshold voltage and to the insensitivity of the deep interface traps to the gate voltage due to a low emission rate. A typical stretch out of the theoretical C-V curve was obtained only for a MISH structure with a very thin AlGaN layer at 300 °C. We analyzed the experimental C-V characteristics from a SiNx/Al2O3/AlGaN/GaN structure measured at room temperature and 300 °C, and extracted a part of Dit(E). The relatively low Dit (∼1011 eV−1 cm−2) in the upper bandgap indicates that the SiNx/Al2O3 bilayer is applicable as a gate insulator and as an AlGaN surface passivant in high-temperature, high-power AlGaN/GaN-based devices.