GaAs Contact Layer Research Articles

Use of Sn-doped GaAs for non-alloyed ohmic contacts to HEMTs

Segregation of Sn to the surface of MBE grown n+-GaAs layers allows fabrication of non-alloyed Ti/Pt/Au, Al or Ti/W ohmic contacts with low specific contact resistivities (1.1 × 10-6 Ω·cm2). These contacts were used to realise high performance HEMTs (gm = 230 mS/mm for 1.0 µm gate length) in which Si is used as the dopant in the donor AlGaAs layer and Sn is employed in the GaAs contact layer.

Electron transport across a wide AlGaAs barrier

An experimental and theoretical study has been made of electron transport over a wide AlGaAs barrier with graded interfaces sandwiched between GaAs contact layers. The width of the central barrier region was varied between 700 and 2100 Å. Two series of samples with nominally identical structures but from different sources were investigated. Extensive measurements of both the voltage and temperature dependence of the current were made, as well as measurements of capacitance and magnetoresistance. Drift-diffusion thermionic emission theory has been used to interpret the data. Both numerical and analytical solutions of the model have been developed and were found to be in good agreement with each other. The presence of space charge in the barrier region, which has the effect of increasing the barrier height, was seen to be crucial to an understanding of the data. When the effect of space charge was included in the model good agreement was obtained between theory and experiment for electric fields up to 10 kV cm−1. The numerical solution required only one adjustable parameter, namely the value of the space-charge density. The parameters used in the analytical model were all derived from the experimental data.

Rapid thermal annealing of magnesium implanted GaAs-GaAIAs heterostructures experimental and simulated distributions

The use of rapid thermal annealing (RTA) techniques to anneal ion implanted GaAs compounds is expected to have a significant impact on device technology. Due to the short duration of the heat treatment, the implanted impurities may be activated without significant diffusion. For heterojunction bipolar transistor (HBT) applications, high doses of p-type impurities are required to compensate the doping levels of N-GaAlAs emitter and n+ GaAs contact layers. Multi-implantations were chosen to maintain a flat profile down to the base layer. Energies of 30, 60, 150, and 340 keV with doses of 6 × 1013, 9 × 1013,6 × 1014, and 9 × 1014 cm−2, respectively, have been used. Annealing cycles with time durations of a few seconds and temperature in the range of 850–950°C are described. Electrical properties of the annealed samples have been investigated using an electrochemical measurement technique. It was found that hole concentrations as high as 4 × 1019 cm−3 and electrical activities near to 75 percent can be obtained. There is no evident indiffusion and no significant outdiffusion at the optimal annealing conditions. Simulation of multilayer implantations are also carried out by an accurate model available in TITAN 2D process simulator using Pearson IV laws and taking into account the diffusion effects on profile distribution caused by RTA. A first approximation using a simple model allows a rapid evaluation of the data fitting operation. In a second approach, concentration dependent diffusivity and the contribution of the electric field at the interface are covered to perform an improved data fitting of ion implanted and annealed dopant profiles. A comparative study shows a good agreement between experimental and simulated distributions.

A MBE-grown high-efficiency GaAs solar cell with a directly deposited aluminum front contact

A novel metallization scheme for GaAs p-n solar cells grown by molecular beam epitaxy (MBE) is described. A p/sup +/-GaAs contact layer was grown on top of the AlGaAs window layer, followed by a pure aluminum layer. This MBE-grown aluminum layer serves both as metallization and as self-aligned mask for selective etching down to the AlGaAs window. The solar cell showed an efficiency of about 20%; the I-V characteristics revealed that negligible series resistance was present in the structure. To test the novel p-contact a second sample was grown. Using a transmission-line model (TLM) structure, a metal-semiconductor contact resistance of 1.5*10/sup -2/ Omega -cm/sup 2/ was found. >

Electroluminescence investigations of electron and hole resonant tunneling in p-i-n double-barrier structures.

The electroluminescence and current-voltage characteristics of a p-i-n double-barrier structure based on GaAs/AlAs are investigated. Electroluminescence lines due to carrier recombination in the GaAs contact layers and in the quantum well are observed. The bias dependence of the intensity of these lines exhibits the pronounced peaks that are also seen in the I(V) characteristics, which are due to electron and hole resonant tunneling. The quantum-well emission lines correspond to recombination of holes in the two lowest-energy valence subbands (LH1 and HH1). Their relative intensities indicate that the hole population in these subbands is inverted over a wide range of bias. A rapid cooling of the holes is observed when the electron density in the quantum well is high.

Light-activated resistance switching in GaAs/AlGaAs naturally-occurring nanostructures

Light-activated discrete resistance switching is studied in GaAs/AlGaAs naturally-occurring nanostructures. These nanostructures consist of small-area current paths through a 0.5 μm thick Al 0.5Ga 0.5As barrier which is sandwiched between two n + GaAs contact layers. The small-area current paths are produced by dislocations which thread through the barrier. Data is presented from one device in which a persistent, > 10 7 increase in current is observed after exposure to light. The current undergoes this transition in ∼ 10 3 discrete steps. Each discrete increase in current corresponds to the photo-population of individual traps present in the barrier material.

Tunneling of minority holes through a double-barrier resonant-tunneling structure under applied bias

Time-resolved photoluminescence of an AlAs/GaAs double barrier resonant tunneling structure has been measured under bias in the region of the first electron resonance, both for exciton recombination inside the quantum well, and in the n-type GaAs contact layers. The characteristic time evolution of the confined exciton and n-GaAs emission reveals the processes of accumulation of photocreated holes and of sequential hole tunneling through the first and second AlAs barrier, for which nearly equal tunneling times are observed. The hole tunneling time is found to decrease monotonically with increasing voltage, as expected for tunneling through a single barrier.

Photodetectors fabricated on heteroepitaxial GaAs/Si structures grown by molecular beam epitaxy

Photodetectors were fabricated on GaAs/Si epitaxial structures grown on high resistivity Si substrates by modulated molecular beam epitaxy. Our efforts were focused on enhancing the substrate surface quality and optimizing the deposition conditions during the early stages of growth of the GaAs on the Si substrates. The photodetector structures investigated consisted of a 300Å thick GaAs/AlAs nucleation layer, a 5-period GaAs (100Å)/AlAs(100Å) accomodation layer grown at 300°C, two GaAs/InGaAs strained layer superlatices separated by a GaAs spacer layer, and a 2μm undoped GaAs active photodetector layer, a 0.2μm n-Gaas and a n +-GaAs contact layer. Various photodetector configurations fabricated in these GaAs/Si material structures were investigated. The photodetector response measurements were made using a 840nm wavelength pulsed laser with a pulse width of 5ns and rise (t r) and fall (t f) times of 200ps. Typical rise and fall times of the photodetectors were in the 1–2 ns and 3–6 ns ranges, respectively. The responsivity and quantum efficiency values of these photodetectors were in the ranges of 0.5–1.0 A/W and 0.3–1.5, respectively.

Solid-phase epitaxial PdGe ohmic contacts to resonant tunneling diodes with thin contact layers

Solid-phase epitaxial PdGe ohmic contacts were used on molecular beam epitaxial grown AlAs-GaAs-AlAs resonant tunneling diodes. In order to probe the vertical extension of the PdGe contact scheme, the contact was made directly on the lowly doped (5 10 16 cm −3) GaAs spacer layer ranging in thickness from 2 to 100 nm. Current-voltage characteristics showed good resonant tunneling performance for structures with contact layers as thin as 12 nm. No resonant tunneling features could be observed for the structure with a 2 nm contact layer. The abruptness of the Ge/GaAs heterojunction and the GaAs consumption during the PdGe contact formation was investigated by high resolution cross-sectional transmission electron microscopy (HREM). This analysis showed that about 4 nm of GaAs is consumed during the PdGe contact formation. For the sample with the 2 nm GaAs contact layer, the top GaAs is consumed together with the first AlAs barrier, which explains the observed lack of negative differential resistance.

Non-linear optical rectification at 10.6 mu m in compositionally asymmetrical GaAs/AlGaAs multi-quantum wells

Summary form only given. The authors present evidence of nonlinear optical rectification in compositionally asymmetrical GaAs/AlGaAs MQWs (multi-quantum wells). The structure consisted of 12 periods of 30-AA GaAs-65-AA Al/sub 0.2/Ga/sub 0.8/As wells separated by 500-AA Al/sub 0.4/Ga/sub 0.6/As barriers, epitaxially grown on a 3*10/sup 18/ cm/sup -3/ Si-doped GaAs wafer. The GaAs well is 3*10/sup 17/ cm/sup -3/ Si doped, while the rest of the wells and barriers are nonintentionally doped. A 3000-AA 10/sup 18/-cm/sup -3/ Si-doped GaAs contact layer is grown. The optical rectification is measured at 77 K as a bias appearing at the diode electrodes (with no photocurrent) when it is illuminated by a 10.6- mu m CO/sub 2/ laser. An optical rectification coefficient chi ( omicron = omega - omega ) of 5*10/sup -6/ m/V for each well was determined that is an effective electrooptical coefficient of 4.5*10/sup -9/ m/V in the whole structure. This value of chi ( omicron = omega - omega ) is more than three orders of magnitude higher than in a natural nonlinear medium such as GaAs or InP at 10.6 mu m. This structure can thus be used as a highly effective electrooptic medium in infrared waveguides. >

Anomalous temperature variation of the thermal resistivity of GaAs IMPATT diodes

The thermal resistivity of GaAs IMPATT diodes was measured from 100-325 degrees C and showed an unexpected 10% decrease, in contrast with the behavior of GaAs MESFET devices, whose resistivity increases as much as 30% over the same temperature range. The most likely explanation of this unusual effect is the dominance of charged impurity scattering in a degenerately doped GaAs contact layer. This may be expected to occur in other devices where heat flow crosses highly doped regions. >

High doping level by rapid thermal annealing of Mg-implanted GaAs/GaAlAs for heterojunction bipolar transistors

Rapid thermal annealing (RTA) of Mg-implanted GaAS/ GaAlAs to contact the base layer of heterojunction bipolar transistors (HBT's) is reported. Annealing cycles with time durations of a few seconds and temperature in the range of 850-950°C have been tested. Electrical properties of the annealed samples have been investigated using an electrochemical measurement technique. It was found that hole concentrations as high as 4 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">19</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> and electrical activities up to 70 percent can be obtained when high doses of Mg are implanted. This high doping level is very important since the final objective is to realize a contact region to the p-type base layer of HBT's through a highly doped n+ GaAs contact layer.

Growth of millimeter-wave GaAs IMPATT structures by molecular beam epitaxy

The technique of molecular beam epitaxy (MBE) was applied to the growth of millimeter‐wave GaAs double‐drift, flat‐profile IMPATT structures to meet the stringent doping concentration and thickness requirements. Each structure has five epitaxial layers and was prepared by first growing an undoped AlxGa1−xAs (x∼0.3) layer on a Cr‐doped, semi‐insulating GaAs(100) substrate held at 600 °C, followed by an n+‐GaAs (>1018 cm−3) buffer layer, and n‐GaAs (>1017 cm−3) layer, a p‐GaAs (>1017 cm−3) layer, and a p+‐GaAs (>1018 cm−3) contact layer. Si and Be were used as n‐type and p‐type dopants, respectively. The AlxGa1−xAs layer acts as an effective ‘‘etch stop’’ during device fabrication, allowing selective removal of the substrate from the epitaxially grown layers in order to reduce the device series resistance. Each device was fabricated with its own ‘‘integral packaging’’ and oscillated in microstrip resonators. The cw oscillations were obtained up to 109 GHz, which is the highest frequency achieved with double‐drift‐type GaAs IMPATTs.

A study of high-speed normally off and normally on Al0.5Ga0.5As heterojunction gate GaAs FET's (HJFET)

The dc, small-signal microwave, and large-signal switching performance of normally off and normally on Al 0.5 Ga 0.5 As gate heterojunction GaAs field-effect transistors (HJFET) with submicrometer gate lengths are reported. The structure of both types of devices comprises an n-type 1017-cm-3Sn-doped active layer on a Cr-doped GaAs substrate, a p-type 1018-cm-3Ge-doped Al 0.5 Ga 0.5 As gate layer and a p+-type 5 × 1018-cm-3Ge-doped GaAs contact and cap layer on the top of the gate. The gate structure is obtained by selectively etching the p+-type GaAs and Al 0.5 Ga 0.5 As. Undercutting of the Al 0.5 Ga 0.5 As layer results in submicrometer gate lengths, and the resulting p+-GaAs overhang is used to self-align the source and the drain with respect to the gate. Normally off GaAs FET's with 0.5- to 0.7-µm long heterojunction gates exhibit maximum available power gains (MAG) of about 9 dB at 2 GHz. Large-signal pulse measurements indicate an intrinsic propagation delay of 40 ps with an arbitrarily chosen 100-Ω drain load resistance in a 50-Ω microstrip circuit. Normally on FET's with submicrometer gate lengths (∼0.6 µm) having a total gate periphery of 300 µm and a corresponding dc transconductance of 20-30 mmhos exhibit a MAG of 9.5 dB at 8 GHz. The internal propagation delay time measured under the same conditions as above is about 20 ps.

Normally-off Al0.5Ga0.5As heterojunction-gate GaAs f.e.t.

D.C. microwave and large-signal switching properties of a normally-off heterojunction-gate GaAs f.e.t. are reported. The device structure comprises an n-type active channel layer, a p-type Al0.5Ga0.5 As gate layer and a p+-type GaAs contact layer. The gate structure is obtained by selectively etching the p-type GaAs and Al0.5Ga0.5As. Undercutting of the Al0.5Ga0.5 As layer results in a submicrometre gate length, and the resulting p+-GaAs overhang is used to self align the source and the drain with respect to the gate. GaAs f.e.t.s with 0.5 to 0.7μ-long heterojunction gates have exhibited maximum available power gains of about 9 dB at 2 GHz. Large-signal pulse measurements indicate an intrinsic propagation delay of 40 ps with an arbitrarily chosen 100Ω drain load resistance in a 50Ω microstrip circuit.