Undoped Barrier Research Articles

Influence of Overheating Effect on Transport Properties of AlGaN/GaN Heterostructures

ABSTRACTAlGaN/GaN heterostructures can operate at very high power in dc and rf regimes due to the superior properties of III-nitride materials. At the same time, the higher level of dissipated power causes higher overheating in the structures. Considering the fact that the temperature is a driving force for aging processes in devices, it is clear that the investigations of thermal effects in 2DEG of GaN-based structures are of great importance. Additionally, such studies result in a better understanding of the transport features which affect device performance, and allow us to determine the conditions under which hot-electron effects play the dominant role in electron transport. In this report we focus on thermal effects in transmission line model (TLM) patterns. The experimental results and numerical simulations of self-heating effects in TLM patterns processed on heterostructures with different layer design (different Al mole fraction of barrier layer, different substrates) are analyzed. The GaN-based heterostructures were grown by MOCVD. One of the investigated samples consists of a nucleation layer of AlGaN (16% Al), an undoped GaN buffer layer and an AlGaN (33%) undoped barrier layer. This type of structure was formed on sapphire and SiC substrates. Additionally, another two types of structures were grown on sapphire substrate: with wide bandgap barrier layer (with Al mole fraction of 75%), and with a thin AlN spacer layer (5nm) in between the AlGaN barrier layer (23 nm) and the GaN buffer layer (1300 nm). All samples were processed with TLM patterns with standard Ti/Al/Ti/Au contact metallization and annealed for 40s at 800° C. The conducting channel of the devices has a width W = 100 μm with intercontact lengths L of 1, 5, 10, 15, 20, 25, 30, 35 μm. The average overheating temperature over the active channel area for each structure type is estimated. The strong influence of the Al mole fraction of the barrier layer and also the substrate type of the structure on noise and transport properties is revealed and analyzed. It was shown that the implementation of wide bandgap barriers and high thermal conductance substrates allows us to improve the overall performance of the structures significantly. Optimal conditions for the observation of hot-electron effects are determined.

Voltage tunable superlattice infrared detector for mid- and long-wavelength detection

We have designed and fabricated a voltage tunable superlattice (SL) infrared photodetector where the detection wavelength switches from the 3–5μm midwavelength infrared (MWIR) range under negative bias to the 8–12μm long-wavelength infrared (LWIR) range under large positive bias. The structure consists of multiple periods of two different SLs that are separated by undoped blocking barriers on one side and heavily doped layers on the other side. The background-limited temperature with F∕1.2 optics is 110 and 70 K for mid- and long-wavelength detection, respectively. This voltage tunable MWIR/LWIR detector has a performance comparable to those of one-color quantum-well infrared detectors designed for the respective wavelength ranges.

Demonstration of undoped quaternary AlInGaN∕GaN heterostructure field-effect transistor on sapphire substrate

Undoped AlInGaN∕GaN heterostructure field-effect transistors (HFETs) have been demonstrated on sapphire substrate. The maximum drain current of 758mA∕mm and extrinsic transconductance of 123mS∕mm were obtained from the device with 2μm gate length and 15μm gate width. Such performance was comparable to that of a conventional modulation-doped (MOD) AlGaN∕GaN HFET fabricated at same the condition. In spite of the high-density two-dimentional electron gas formed at the heterointerface, the AlInGaN∕GaN HFET structure showed relatively low Hall mobility (689cm2∕Vs at 300K), which was due to the high concentration background doping in the undoped AlInGaN barrier region. Encouragingly, both the gate leakage current and drain current collapse showed lower values than that of the MOD-AlGaN∕GaN HFET, which implied the potential application of quaternary AlInGaN to high-power and high-frequency devices.

Low current dispersion and low bias‐stress degradation of unpassivated GaN/AlGaN/GaN/SiC HEMTs

The performance of unpassivated GaN/AlGaN/GaN/SiC HEMTs with intentionally undoped and doped barrier structures are extensively studied. No drain current dispersion between DC and 50 ns pulses on doped devices and less than 10% dispersion on undoped ones is observed. The full (100%) current recovery on undoped sample was measured in 1 µs. The drain current extrapolated from 2 GHz large signal measurements corresponds to the measured static drain current confirming negligible current dispersion of our devices. Insignificant (<5%) degradation in overall device performance parameters (IDs, gm, fT, fmax, Pout) on both undoped and doped structures after 12-hour-long bias stress was obtained. These results documents that suitable device performances can be obtained also on unpassivated GaN-based HEMTs. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Low Threshold Current Density 1.5 μm Strained-MQW Laser by n-Type Modulation-Doping

1.5µm n-type modulation-doping InGaAsP/InGaAsP strained multiple quantum wells grown by low pressure metalorganic chemistry vapor decomposition technology is reported for the first time in the world. N-type modulation-doped lasers exhibit much lower threshold current densities than conventional lasers with undoped barrier layers. The lowest threshold current density we obtained was 1052.5 A/cm2 for 1000 µm long lasers with seven quantum wells. The estimated threshold current density for an infinite cavity length was 94.72A/cm2/well, reduced by 23.3% compared with undoped barrier lasers. The n-type modulation doping effects on the lasing characteristics in 1.5µm devices have been demonstrated.

InAlAs/InGaAs doped channel heterostructure for high-linearity, high-temperature and high-breakdown operations

High-linearity In 0.52Al 0.48As/In 0.53Ga 0.47As doped channel transistor (DCFET) was successfully fabricated by low-pressure metal organic chemical vapor deposition (LP-MOCVD). Due to the presence of doped InGaAs channel, undoped InAlAs barrier layer and low-temperature growth InAlAs buffer layer, good carrier confinement, improved breakdown and a wide transconductance operation regime are expected. Experimentally, for a 1.5 × 100 μm 2 device, high-transconductance of 291 mS/mm, high-gate-drain breakdown voltage of 20.2 V, high-turn-on voltage of 0.78 V, wide operation regime of 288 mA/mm as well as significantly improved gate voltage swing of 1.05 V are achieved. Additionally, the studied device shows good temperature characteristics. The performance indicates the promise for high-power and high-temperature applications.

Inclusion of Silicon Delta-doped Two-dimensional Electron Gas Layer on Multi-quantum Well Nano-structures of Blue Light Emitting Diodes

The influence of heavily Si impurity doping in the GaN barrier of InGaN/GaN multi-quantum well structures of blue light emitting diodes were investigated by growing samples in metal-organic chemical vapor deposition. The delta-doped sample was compared to the sample with the undoped barrier. The delta-doped sample shows the tunneling behavior and forms the energy level of 0.32 eV for tunneling and the photoemission of the 450-nm band. The photo-luminescence shows the blue-shifted broad band of the radiative transition due to the inclusion of Si delta-doped layer indicating that the delta doping effect acts to form the higher energy level than that of quantum well. The dislocation may provide the carrier tunneling channel and plays as a source of acceptor. During the tunneling of hot carrier, there was no light emission.

Fabrication of high-performance 370 nm ultraviolet light-emitting diodes

High-performance 370 nm ultraviolet light-emitting diodes (UV-LED) with an AlGaN/InGaN single quantum well structure were fabricated on sapphire substrate by using the metal organic chemical vapor deposition (MOCVD) technique. Three group different growth conditions of undoped n-side barrier and p-side barrier were chosen to sandwich the InGaN well layer in the LED samples. By using high-Al content and symmetric composition n- and p-side barriers, the output efficiency of our UV-LED has been greatly improved. An output power of 2.5 mW at an injection current of 20 mA was achieved.

Electrical properties and luminescence spectra of light-emitting diodes based on InGaN/GaN heterostructures with modulation-doped quantum wells

The distribution of charged centers N(w), quantum efficiency, and electroluminescence spectra of blue and green light-emitting diodes (LED) based on InGaN/AlGaN/GaN p-n heterostructures were investigated. Multiple InGaN/GaN quantum wells (QW) were modulation-doped with Si donors in GaN barriers. Acceptor and donor concentrations near the p-n junction were determined by the heterodyne method of dynamic capacitance to be about N A ≥ 1 × 1019 cm−3 ≫ N D ≥ 1 × 1018 cm−3. The N(w) functions exhibited maxima and minima with a period of 11–18 (±2–3 nm) nm. The energy diagram of the structures has been constructed. The shifts of spectral peaks with variation of current (J=10−6–3×10−2 A) are smaller (13–12 meV for blue and 20–50 meV for green LEDs) than the corresponding values for the diodes with undoped barriers (up to 150 meV). This effect is due to the screening of piezoelectric fields in QWs by electrons. The dependence of quantum efficiency on current correlates with the charge distribution and specific features in the current-voltage characteristics.

Microstructural and optical properties of InAs/GaAs quantum dots embedded in modulation-doped AlxGa1−xAs/GaAs heterostructures

The microstructural and the optical properties of InAs/GaAs quantum-dot (QD) arrays inserted into undoped GaAs barriers embedded in an AlxGa1−xAs/GaAs were investigated by using transmission electron microscopy (TEM) and photoluminescence (PL) measurements. The TEM images and the selected-area electron diffraction patterns showed that vertically stacked InAs QD self-assembled arrays were embedded in the GaAs barriers. The temperature-dependent PL spectra showed that the peak corresponding to the interband transitions from the ground electronic subband to the ground heavy-hole band of the InAs QDs shifted to lower energy with increasing temperature. The PL intensity of the InAs dots was significantly enhanced by the modulation-doped AlxGa1−xAs/GaAs heterostructure, and the thermal activation energy of the InAs dots was decreased by the addition of the modulation-doped AlxGa1−xAs/GaAs heterostructure. The present results can help to improve the understanding of the microstructural and the optical properties in InAs QD arrays inserted into GaAs barriers embedded in a modulation-doped AlxGa1−xAs/GaAs heterostructure.

Characterization of InP/InGaAs double-heterojunction bipolar transistors with tunnelling barriers and composite collector structures

We study and demonstrate the dc performance of two InP/InGaAs double-heterojunction bipolar transistor (DHBTs) with the undoped tunnelling barrier and composite collector structures. Due to the mass filtering effect for holes, a thin InP tunnelling barrier can be used to replace the wide-gap emitter. By varying the thickness of the barrier, distinct collector current ideality factors can be obtained which reveal different injection mechanisms at the emitter. The 4000 Å InP collectors with InP/InGaAs abrupt junctions and InP/InGaAsP/InGaAs step-graded junctions achieve high breakdown voltages of 9.2 and 14.6 V, respectively. Furthermore, the abrupt junction and δ-doping structure eliminate carrier blocking across the base-collector heterojunction more effectively than the step-graded junction. We find that the reduction of the multiplication avalanche of the step-graded junction DHBT leads to the severe self-heating effect. For the abrupt junction DHBT, the dc current gain is almost independent of the perimeter-to-area ratio of the emitter due to the low surface recombination.

Microstructural and interband transition studies of multiple stacked layers of Si-doped InAs self-assembled quantum dot arrays inserted into GaAs barriers

Microstructural and optical properties of Si-doped InAs quantum dot (QD) arrays inserted into undoped GaAs barriers have been investigated by using energy dispersive X-ray fluorescence (EDX), transmission electron microscopy (TEM), and photoluminescence (PL) measurements. The EDX pattern, the TEM image, and the selected area electron diffraction pattern showed that self-assembled Si-doped InAs vertically stacked QD arrays were embedded in the GaAs barriers. The temperature-dependent PL spectra showed that the peaks corresponding to the interband transitions of the InAs QDs shifted to the low-energy side with increasing temperature and that the distribution of carriers in the InAs QDs varied with changing temperature. These results indicate that Si-doped InAs QD arrays inserted into GaAs barriers hold promise for potential applications in optoelectronic devices.

Current instabilities in GaN-based devices

Current dispersion effects have been experimentally investigated in a variety of AlGaN/GaN heterostructure FETs with large signal and switching measurements including HEMTs with doped and undoped barrier layer. A range of dispersion frequencies from 10/sup -3/ Hz to 10 GHz were observed, where the output current amplitude is drastically reduced. Through this effect the full channel charge of an AlGaN/GaN heterostructure FET may be completely depleted under specific bias conditions. This indicates that this phenomena cannot be related to deep traps alone, but is also connected to piezorelated charge states and conduction to these states.

Shallow donors in ultrathin nitrogen-doped ZnSe layers – a novel or a disregarded compensation mechanism in II–VI device structures?

In the last decade, a manifold of studies has been conducted to elucidate the compensation mechanism in strain relaxed N-doped ZnSe layers of about 1 μm or more thickness. It has mostly been disregarded, however, that in optoelectronic devices often N-doped layers thinner than the critical thickness are used. We have investigated the donor–acceptor-pair (DAP) recombination in N-doped MBE-grown ZnSe layers being as thin as 100, 20, 10, and 5 nm embedded in undoped ZnMgSSe barriers lattice matched to the GaAs substrates. The temperature and intensity dependence of the DAP recombination as well as the observation of two-hole transitions of the nitrogen acceptor on the Se site leads us to the conclusion that a not-yet investigated very shallow donor is involved in the DAP recombination. This impurity with a thermal activation energy of 19±1 meV seems to be the dominant compensating donor in ultrathin fully-strained N-doped ZnSe, at least if embedded in quaternary barriers.

Characterization of Ga 0.52In 0.48P/GaAs single quantum well structures grown by solid source molecular beam epitaxy using deep level transient spectroscopy

A GaInP/GaAs/GaInP single quantum well (SQW) sample with unintentionally doped barrier layers was studied using deep level transient spectroscopy (DLTS). A significant DLTS signal was observed, and believed to be from the thermal emission of the well electrons in the quantum well. Furthermore, it was found that only a single major peak was observed, thus indicating that the GaInP/GaAs/GaInP SQW should be at the single-trap level state. Different combinations of V r (reverse bias) and fill pulse height (FPH) allowed a DLTS study of the region before, within, and beyond the well location. Such an observation, considering the use of an undoped GaInP barrier layer, proves that the DLTS signal is indeed from the well because it is only significant when probed within the well region. The assumption of the DX centers in some previous studies involving the use of AlGaAs barriers can be excluded.

Intersubband absorption in boron-doped multiple Ge quantum dots

The intersubband absorption in self-assembled boron-doped multiple Ge quantum dots is observed. The structures used consist of 20 periods of boron-doped Ge dot layers and undoped Si barriers. The infrared absorption as a function of wavelength is measured by Fourier transform infrared spectroscopy using a waveguide geometry. Absorption peaks in the mid-infrared range have been observed, which are attributed to the transitions between the first two heavy hole states of the Ge quantum dots. The polarization dependence measurement is used to study the nature of the transitions. This observation suggests the possible use of multiple Ge quantum dots for infrared detector application.

Investigation of broadband p-type quantum-well infrared photodetectors

Typical quantum-well infrared photodetectors (QWIPs) exhibit rather narrow spectral bandwidths of 1–2 μm. For certain applications, such as spectroscopy, sensing of a broad range of infrared photons is necessary. In this letter, we report on the design of two p-type broadband (BB) QWIPs for detection in the 7–14 μm long-wavelength infrared band. Two different QWIP structures were designed: QWIP-A with three wells of different InxGa1−xAs(x=0.2,0.25,0.3) compositions and QWIP-B with three wells of In0.25Ga0.75As of different thicknesses (4.5, 5.5, and 6.2 nm) and an undoped GaAs barrier (40 nm thick) to form a unit cell, which are then repeated 20 times to create the BB-QWIPs. The spectral response for QWIP-A covers the wavelengths from the 7 to 11 μm range, while QWIP-B has a broader spectral response bandwidth from 7 to 14 μm. A maximum responsivity of 25 mA/W was obtained for QWIP-B at Vb=1.1 V, λp=10.2 μm, and T=40 K with a spectral bandwidth of Δλ/λ=63%, a cut-on wavelength at 6.2 μm, and a cut-off wavelength around 14 μm.

Doping optimizations for InGaAs/InP composite channel HEMTs

InGaAs/InP composite channel HEMTs are good candidates for electro-absorption modulator driver applications where both high speed and high breakdown voltage are required. For other applications – input stage of a pin-HEMT receiver for example – high cutoff frequency must meet with low leakage current. This calls for low delta-doped or undoped barrier material which is not easily compatible with high speed and high drain current. To achieve simultaneously all these criteria, a new HEMT heterostructure called here TC-HEMT (for triple channel-HEMT) has been investigated. Different HEMT heterostructures have been grown by LP-MOVPE for comparison. In this study, Hall mobilities and sheet carrier densities at 300 K have been measured for each structure and for different delta-doping level and spacer thickness. Finally, we compare static and dynamic characteristics measured on transistors near the Hall patterns.

Effects of Si-doping in the barriers of InGaN multiquantum well purplish-blue laser diodes

Optical gain spectra of InGaN multiquantum well laser diode wafers having Si-doped or undoped InGaN barriers were compared. Although evidence for effective band-gap inhomogeneity was found in both structures, the wells with the Si-doped barriers exhibited a smaller Stokes-like shift. Si doping suppressed emergence of a secondary amplified spontaneous emission peak at 3.05 eV, which was uncoupled with the primary one at 2.93 eV. Furthermore Si doping reduced the threshold power density required to obtain the stimulated emission.

Defect-Enhanced Electron Transport through Semiconductor Barriers

It has often been suggested that defects can enhance electron tunneling through barriers. Here, we derive a general expression of the current flowing through semiconductor barriers when induced by the presence of defects, taking into account tunneling and other possible mechanisms. Therefore, we calculate the capture probability of free electrons by defects located in the barrier and the subsequent emission probabilities of the captured electrons, by thermal emission or phonon-assisted tunneling into the barrier conduction band. The defect potential considered is composed of a δ-function localized on first neighbors and of a Coulomb tail, thus allowing to treat all types of defects from shallow to deep ones. The result of the calculation is then compared with experiment. The temperature and electric field dependences of the current fit well the experimental data obtained for undoped GaAlAs barriers embedded in n+ GaAs, if the defect considered is the DX center. The concentration of these native donor defects that accounts for the amplitude of the current is, as expected, of the order of 1016 cm—3.