Low-resistance ohmic contacts to AlGaAs/GaAs and In0.52Al0.48As/In0.53Ga0.47As modulation-doped structures obtained by halogen lamp annealing

The successful application of short-term halogen lamp annealing to form ohmic contacts to AlGaAs/GaAs and In0.52Al0.48As/ In0.53Ga0.47As modulation-doped structures is demonstrated. Use of Ti in the electron-beam evaporated metallization scheme and a two-step annealing cycle give contacts with reproducibly good electrical and morphological characteristics. Minimum values of specific contact resistancerho_{c} = 4.0 times 10^{-7}and6.0 times 10^{-7}Ω.cm²for AlGaAs/GaAs and In0.52Al0.48As/In0.53Ga0.47As, respectively, are measured. Corresponding values of the transfer resistance Rcare 0.12 ± 0.02 and 0.18 ± 0.05 Ω.mm. These values are the lowest achieved with lamp annealing and are comparable to the best obtained with transient furnace annealing.     

The carrier-induced index change in AlGaAs and 1.3 µm InGaAsP diode lasers

The carrier-induced index change has been measured for a large number of broad-area AlGaAs and 1.3 μm InGaAsP diode lasers. The observed index change with injected carrier density is-(1.2 pm 0.2) times 10^{-20}cm³for AlGaAs lasers and-(2.8 pm 0.6) times 10^{20}cm³for 1.3 μm lasers. The index change at threshold varies from -0.03 to -0.06 for AlGaAs lasers and from -0.04 to -0.10 for InGaAsP lasers. This variation is correlated with the carrier density at threshold, which depends on active layer thickness and doping level. For the first time, the observed index change is compared to the carrier density found from differential carrier lifetime measurements. This accurate determination of the carrier density represents a significant improvement over previous studies.     

InGaAs/InP double-heterostructure bipolar transistors with near-ideal β versus ICcharacteristic

InGaAs/InP double-heterostructure bipolar transistors (DHBT's) with current gain β ∼ 630 have been realized using gas-source molecular beam epitaxy (GSMBE). These devices exhibit near-ideal β versus ICcharacteristic (i.e., β independent of IC) with a small-signal gainh_{fe} sim 180atI_{C} sim 2nA. In comparison, we findbeta sim I_{C}^{0.5}for a high-quality AlGaAs/GaAs HBT grown by OMCVD. The higher emitter injection efficiency at low collector current levels found in the InGaAs/InP DHBT is due to at least a factor 100 smaller surface recombination current.     

Effects of carrier confinement by InGaAs/GaAs heterointerfacebarrier on deep trap concentration profiling

Carrier confinement by the conduction-band barrier formed by the band discontinuity ΔE_c has been observed in molecular beam epitaxy (MBE)-grown In_0.1Ga_0.9As/GaAs heterostructures as evidenced by the decrease of DLTS (deep level transient spectroscopy) peak height with increased filling pulse amplitude in a fixed-reverse-bias variable-filling-pulse DLTS study. A DLTS model including the ΔE_c effects on trap profiling is developed by considering conduction-band barrier as a giant deep trap with an electron emission rate depending exponentially on ΔE_c. The model explains quantitatively the experimental observations of the effects of ΔE_c in the DLTS study and shows that an erroneous trap concentration profile may result from the conduction-band barrier if the standard DLTS model is used. It is shown that a 0.16-eV±0.01-eV electron trap reported to be located only at the heterointerface of In_0.53Ga_0.47As/InP may actually be a deep-trap characteristic of InGaAs with a fairly uniform trap concentration throughout the InGaAs epilayer     

Effects of hot-electron stress on electrical performances in AlGaAs/InGaAs pseudomorphic high electron transistors

The effects of hot-electron stress on electrical properties in AlGaAs/InGaAs pseudomorphic high electron mobility transistors were investigated using current-voltage (I–V), capacitance-voltage (C-V), deep level transient spectroscopy (DLTS), and transconductance dispersion measurements. After hot-electron stress, the three-terminal gate-drain breakdown characteristics were improved and the gate-drain capacitance was decreased even though no difference was found in both DLTS and transconductance dispersion results. These results suggest that hot electrons were trapped at the interface of the passivation layer, Si₃N₄, with AlGaAs, locating between gate and source/drain electrodes, leading to the increase of the depletion region under the ungated region. On the other hand, the two-terminal gate-drain breakdown characteristics were deteriorated by hot-electron stress. This was due to the reduction of the Schottky barrier height.     

Monolithic integration of curved waveguides and channeled-substrate DH lasers by wet chemical etching

Integration of AlGaAs/GaAs curved waveguides and other two-dimensional waveguides with DH lasers and detectors is demonstrated. Devices are fabricated from LPE AlGaAs/GaAs layers by wet chemical etching processes. Differential transfer efficiencies ofeta_{t}= 5percent are routinely achieved in a structure consisting of an integrated laser, a 90° curved waveguide with 150μm radius, and a detector, for the case where one laser mirror is etched and one cleaved. This value iseta_{t}= 4percent if both mirrors are etched. A comparison of waveguide attenuation between straight and curved rib waveguides is given, along with the transfer characteristics of curved waveguides. The loss coefficient of curved rib waveguides with 150-μm radius is about two times that of a straight waveguide of the same length. The fabrication and properties of channeled-substrate crescent (CSC) lasers and detectors with transverse single-mode confinement, monolithically integrated by means of passive CSC interconnecting waveguides, is also described.     

Drain current dlts analysis of recoverable and permanent degradation effects in AlGaAs/GaAs AND AlGaAs/InGaAs HEMT'S

We present a detailed study of drain current DLTS spectra performed on asreceived and failed AlGaAs/GaAs and AlGaAs/InGaAs HEMT's of four different suppliers submitted to hot-electron tests. We demonstrate that a remarkable correlation exists between DLTS features and permanent and recoverable degradation effects. In particular, different behaviours have been found: (i) recoverable effects seems to be correlated with modulation of charge trapped on DX and ME6 centers. (ii) permanent degradation consisting in a decrease in Id and V_T is due to negative charge trapping and is associated with a large increase of a peak having Ea=1.22 eV in the DLTS spectra of failed devices; (iii) development of traps in the gate-to-drain access region induces a permanent increase in drain parasitic resistance Rd and decrease in Id, and is correlated with the growth of a “hole-like” peak in DLTS spectra measured after hot-electron tests.     

Measurement of radiative and nonradiative recombination rates in InGaAsP and AlGaAs light sources

A small signal method is used to measure the carrier lifetime as a function of injected carrier density, and the results are used to determine the radiative and nonradiative recombination rates for AlGaAs LED's and 1.3 μm InGaAsP lasers. For AlGaAs LED's the radiative recombination constant decreases with injected carrier density and the rate equation contains a small nonradiative Cn³term. The low internal efficiency of 1.3 μm InGaAsP lasers is found to be primarily caused by two factors: a radiative coefficientB(n)which strongly decreases with the injected carrier density, and CHHS Auger recombination having a recombination coefficient of1-2 times 10^{-29}cm⁶/s. A recombination term representing carrier leakage is observed in some devices, but it is not the principal cause of low internal efficiency.     

High-performance p-n-p AlGaAs/GaAs heterojunction bipolar transistors: A theoretical analysis

The AlGaAs/GaAs P-n-p heterojunction bipolar transistor (HBT) is shown by a simple analysis to exhibit millimeter wave and digital switching performance comparable to similar N-p-n structures. For example, a P-n-p HBT with a 1-µm emitter stripe and 34-µm²total area yieldsf_{tau} = 31GHz,f_{max} = 94GHz, and an intrinsic switching speedtau_{s} = 8ps. A similar N-p-n structure exhibitsf_{tau} = 56GHz,f_{max} = 102GHz, andtau_{s} = 8ps.     

AlGaAs defect characterization in high electron mobility transistors by thermally stimulated drain conductance

A method based on monitoring the thermally stimulated drain conductance (TSDC) under trap filling and emptying bias-temperature sequences is developed for directly characterizing high defect density AlGaAs layers in high electron mobility transistors (HEMT's). The rate equation for trapped electron emission is solved for the conditions pertinent to the TSDC method. Measured results on HEMT's are in excellent agreement with this solution and compare favorably with values reported by others. TSDC is as convenient to implement but more applicable than deep-level transient spectroscopy (DLTS) for the high concentration of AlGaAs defects normally encountered in HEMT's.     

Low-frequency response to AlGaAs double heterojunction LED's

At low frequencies, the optical response of a light emitting diode (LED) to analog intensity modulation is found to decrease, due to the junction-heating effects. This low-frequency response is analyzed using approximated I-P characteristics, and is compared with the experimental results for AlGaAs double heterojunction LED's. The transfer function of the low-frequency response can be expressed by1 - (1- phi)/(1+ jomegatau_{sigma}), where φ and tauσare the degradation of the fundamental amplitude at lower frequencies normalized to higher frequencies and the thermal time constant with a value of about 10 µs, respectively. It is found that the low-frequency response correlates with the linearity of the I-P characteristics, using the low-frequency degradation factorpsiexpressed by-10 log(1- phi)in decibels and the linearity coefficient γ defined as{P(1.5 I{o}) - P(I_{o})}/{P(I_{o}) - P(0.5 I_{o})}, whereP(1.5 I_{o})denotes the power at 1.5 times of the basic current Io. Also, the current dependence and modulation degree dependence on both degradation factorpsiand second-harmonic distortion are discussed.     

Direct measurement of the potential spike energy in AlGaAs/GaAs single-heterojunction bipolar transistors

A balanced two-step current transport theory, i.e., thermionic emission followed by Shockley diffusion, is applied to study the emitter-base (EB) potential spike energy in the AlGaAs/GaAs single-heterojunction bipolar transistor. It is found, surprisingly, that when the transistor is operated in the active region theI-Vcharacteristics of the collector current (IC) versus base-emitter applied voltage (VBE) exhibits an ideality factor of 1.237. This non-1kT transfer characteristics is due to the bias-dependent potential spike energy at the emitter-base heterojunction. The reverse I-V characteristics of emitter current (IE) versus base-collector bias (VBC), however, shows the traditional 1kT behavior. The difference between ICand IEat the same applied voltage (V_{BE} = V_{BC}) determines the potential spike energy (ΔE). It turns out that Δ E/q = 0.19(V_{BE} - 0.48)whereqis the unit charge. This indicates that the potential spike appears only when the applied voltageV_{BE} > 0.48V.     

High-transconductance p-channel modulation-doped AlGaAs/GaAs heterostructure FET's

p-channel modulation-doped AlGaAs-GaAs heterostructure FET's (p-HFET's) employing two-dimensional hole gas (2DHG) were fabricated under various geometrical device parameter conditions. The p-HFET characteristics were measured at 300 and 77 K for the following three device-parameter ranges: the gate length Lg(1-320 µm), the gate-source distance Lgs(0.5-5 µm), and the layer thickness dt(35-58 nm) of AlGaAs beneath the gate. Based on the obtained results, a high-performance enhancement-mode p-HFET was fabricated with the following parameters:L_{g} = 1µm,L_{gs} = 0.5µm, andd_{t} = 35nm. The achieved extrinsic transconductance gmwas 75 mS . mm^-1at 77 K. This experimental result indicates that a gmgreater than 200 mS . mm^-1at 77 K Can be obtained in 1-µm gate p-HFET devices.     

A treatise on the capacitance—Voltage relation of high electron mobility transistors

A new model for the capacitance-voltage relation of a HEMT is presented. The model uses three physically motivated capacitive terms in series. TheC(V)expressions are derived using the quantum mechanical triangular potential well model and the two-dimensional electron-gas charge-control model. These expressions provide further physical insight into the AlGaAs heterosystem. The results obtained should be readily applicable to such techniques as DLTS and otherC(V)interface measurement methods. The equations derived will also serve as a basis for analytical and circuit modeling of HEMT structures.     

Analyses of mode-hopping phenomena in an AlGaAs laser

Intensity fluctuations of the longitudinal modes of a 0.8 μm AlGaAs laser were precisely measured during the occurrence of hopping between two modes. It was found from this result that mode hopping follows the stochastics of a Poisson process. The frequency of mode hopping was measured asf_{c} = [exp [-95(I/I_{th} - 1)]] times 10^{7}(Hz). whereI/I_{th}is the injection current normalized to its threshold value. Results of analog computer simulations showed that spontaneous emission worked as a triggering force for mode hopping. Results of the analysis based on the Fokker-Planck equation were compared to the experimental results, from which the root-mean-square value of the fluctuating electric field of spontaneous emission was estimated as2.3 times 10^{2)(V/m)leqlanglesim{E}_{N} leq 3.2 times 10^{2}(V/m). It is concluded that an effective reduction of mode hopping is achieved if the laser is operated at a higher bias or if the coupling constant between the two modes is increased.     

Reduction of extrinsic base resistance in GaAs/AlGaAs heterojunction bipolar transistors and correlation with high-frequency performance

Improved high-frequency performance in GaAs/AlGaAs heterojunction bipolar transistors (HBT's) by reduction of extrinsic base resistance is demonstrated. A new self-aligned process which is very simple, yet capable of producing 0.25-µm emitter-to-base contact gaps, is described. By the use of AuBe, we have also been able to produce contact resistances to p-type GaAs (p = 5 × 10¹⁸) as low as 1.2 × 10^-7Ω.cm². This is the lowest value reported to p-type GaAs considering the relatively low doping levels used. By employing these techniques, we have produced HBT's with 2.5-µm-wide emitters having current gain cutoff frequencies fTthat appear to be greater than 35 GHz and maximum oscillation frequenciesf_{max}of 22 GHz.     

Low-threshold current AlGaAs/GaAs-distributed feedback laser grown by two-step molecular beam epitaxy

AlGaAs/GaAs-distributed feedback (DFB) lasers with oxide-stripe structure were fabricated by a two-step molecular beam epitaxial (MBE) growth for the first time. The large coupling coefficient of 90 cm^-1and the threshold current as low as 165 mA at room temperature were obtained with the second-order gratings. The characteristic temperature T0was as high as 210 K. Single longitudinal-mode oscillation was observed up toI/I_{th} = 1.5and from 0 to 50°C without any mode hopping. The wavelength variation from device to device was ±5 Å. The dependence of the coupling coefficient on the device structure was calculated, and it was shown that MBE is much more advantageous than LPE to enhance the coupling coefficient.     

On the estimation of base transit time in AlGaAs/GaAs bipolar transistors

Electron diffusion across quasi-neutral p-type base regions representative of those used in n-p-n AlGaAs/GaAs/GaAs heterojunction bipolar transistors is investigated. Monte-Carlo simulation results demonstrate that for realistic base widths ≲ 1000 Å) electron transport cannot be described by Fick's Law. As a result, the conventional estimate of base transit time,tau_{B} = Wmin{B}max{2}/2D_{n}, will produce substantial errors for base widths typical of those employed in heterojunction bipolar transistors. Estimates based on the ballistic transport of electrons across the base are shown to significantly underestimate base transit time-even for base widths substantially narrower than those presently employed.     

Deep center passivation in 3C-SiC by hydrogen plasma with a grid for damage suppression

In order to passivate deep levels, hydrogen was introduced into 3C-SiC by a hydrogen plasma treatment. The effects of hydrogen were investigated by deep level transient spectroscopy (DLTS). DLTS results indicated that hydrogen passivates deep levels in the as-grown sample. However, the results also indicated that the plasma treatment simultaneously forms deep levels originated from damages in the surface region. In order to suppress the damages, we introduced a grid between the sample and the plasma source during the plasma treatment. DLTS results showed that the grid reduces the deep levels originated from the damages, retaining the hydrogen passivation effects. The defect concentration is less than 10¹⁴ cm⁻³ for the sample treated by the hydrogen plasma with the grid and subsequently annealed at 300 °C.     

Deep-level transient spectroscopy measurements using high Schottky barriers

This paper describes a method to investigate minority-carrier traps in semiconductors by utilizing high Schottky barrier diodes in DLTS (deep-level transient spectroscopy) measurements.

This offers an advantage compared to using pn diodes in that no excessive heat treatment which can alter the semiconductor deep-level properties has to be performed.

One of the best-known deep-level systems is the gold levels in silicon. The method is demonstrated for the gold donor level in n-type silicon by using iridium as Schottky contact.