Superlattice Barriers Research Articles

Optimization of charge transfer to the active channel in δ-doped heterostructures

An accurate theoretical model based on the two-band effective-mass-approximation that includes nonparabolic, strain, finite temperature, many-body, and deep donor (DX) center effects is used to investigate the electronic properties of δ-modulation-doped semiconductor heterostructures with the aim of optimizing the active channel density. Inclusion of the DX centers in the model leads to the saturation of the electronic density with increasing δ-doping concentration for both structures doped on one side and structures doped on both sides of the channel. The saturation value in the latter case is almost twice as high as in the former. The self-consistent calculations show that by using a superlattices of superlattices configuration with an appropriately chosen superlattice barrier one can achieve a 50% increase in the maximum charge transfer compared to conventional heterostructures of similar design, without increasing impurity scattering in the channel.

Gas source molecular beam epitaxial growth and characterization of 600–660 nm GaInP/AlInP double-heterostructure lasers

Gas source molecular beam epitaxy (GSMBE) is one of the most promising technologies for growing AlGaInP which is an important compound system for realizing high performance 550–700 nm range visible light emitting devices. In this paper we review the investigations carried out for GSMBE-grown AlGaInP crystals and visible lasers, involving several results for AlGaInP systems grown by metal-organic chemical vapour deposition. First of all, we investigated growth conditions of high optical-quality GaInP by GSMBE, followed by the fabrication of 600 nm range GaInP/AlInP double-heterostructure lasers. To improve the lasing performances, various techniques described in this paper have been investigated. (GaInP) 2.5/(AlInP) 2.5 short-period superlattices were utilized to suppress the non-radiative recombination at the interface between the active and AlInP cladding layers. These superlattices were also used as the superlattice barriers in multiple-quantum-well active layers. The effect of misoriented GaAs substrates from the (100) surface toward the [011] direction on material properties and lasing performances was systematically investigated. It was clarified that the spontaneous crystal ordering was suppressed and the Al-related non-radiative recombination was reduced. Also, this substrate misorientation effect was effective in enhancing the electrical activity of Be in AlInP cladding layers with a resultant drastic reduction in threshold current density. The introduction of novel multiple-quantum-barrier structure into lasers was also investigated, for the first time; this was very effective in reducing the carrier overflow from the active layer. As a result, a drastic improvement in lasing performance of 660 nm lasers was obtained. Finally, a new shutter control method to form a strained quantum well layer without growth interruption is presented.

Optical absorption of the intersubband transitions in GaAs/Al0.4Ga0.6As multiple quantum wells with superlattice barriers

The optical absorption of intersubband transitions in n-type GaAs/Al0.4Ga0.6As multiple quantum wells with superlattice barriers is studied as a function of temperature. The room-temperature spectrum shows a strong intersubband transition at 118.64 meV (10.45 μm) and a shoulder at 137.00 meV (9.05 μm). A broad transition is also observed at 204.6 meV (6.06 μm). The 118.64 meV intersubband transition exhibits a blue shift (∼5.33 meV) as the temperature is decreased from 295 to 4.2 K. The intensity of this transition is increased dramatically while the intensity of the shoulder observed at 137.00 meV is reduced as the temperature is decreased. Theoretical calculations based on a self-consistent k⋅p model which include many-body effects of Hartree, exchange-correlation, depolarization, and vertex effects indicate that the 118.64 meV intersubband absorption is due to the electronic transition between the ground and excited states in the well. On the other hand, the shoulder seems to correspond to a transition between the ground state and the miniband formed in the superlattice barrier.

Realization of high performance doped-channel MISFETs in highly strained AlGaAs/InGaAs/AlGaAs based quantum wells

Doped channel metal-insulator-semiconductor field effect transistors (DC-MISFETs) in the highly strained Al x Ga 1- x As/ In 0.26Ga 0.74As/Al y Ga 1- y As (0< x, y≤1.0) and ((AlAs) m (GaAs) n )/In 0.26Ga 0.74As/( (AlAs) p (GaAs) q ) single quantum wells grown via MBE on GaAs (100) substrates have been examined with a view towards improved performance via control of growth using RHEED behavior of static and dynamic surfaces. Breakdown voltages over 20 V and over 40 V have been realized in structures containing AlGaAs alloy and equivalent Al content short period superlattice (SPSL) barriers, respectively. Low gate leakage currents and output conductance as low as 150 μS/mm have been realized. Increasing the In content to 30% and using heavy Si doping in the well, DC-MISFETs containing SPSL barriers have yielded drain currents of 1.2 A/mm and breakdown voltages of ∼10 V.

Long wavelength quantum well lasers with InGaAs/InP superlattice optical confinement and barrier layers

Long wavelength, separate confinement heterostructure, ridge waveguide laser diodes using InGaAs/InP short period superlattices and InGaAs quantum wells have been demonstrated. The short period superlattices replace the more conventional quaternary alloys. These lasers have threshold current densities of 1.9kA/cm2, device efficiencies of 0.16mW/mA and a characteristic temperature T0 of 56 K.

Low dark current step-bound-to-miniband transition InGaAs/GaAs/AlGaAs multiquantum-well infrared detector

A new step-bound-to-miniband (SBTM) transition multiquantum-well long-wavelength infrared photodetector (LWIP) using a lightly strained In0.07Ga0.93As quantum well and a short-period GaAs/Al0.4Ga0.6As superlattice barrier structure has been developed. The new structure created a potential ‘step’ in the superlattice barrier to block the undesirable electron tunneling current from the heavily doped ground state in the quantum well, which results in a significant reduction in the device dark current. The measured absorbance spectra and photocurrent are in good agreement with our theoretical predictions. The peak detectivity D* at λ=10.5 μm was found to be 2.1×1010 cm √Hz/W at Vb=5 V and T=63 K.

(InAs)1/(GaAs)n Superlattice Quantum Well Lasers

Strained layer In0.2Ga0.8As/GaAs quantum-well (QW) lasers have been used to pump Erbium Doped Fiber Amplifiers for use in opto-electronic communication systems. These InGaAs/GaAs Q-W lasers grown as graded index separate confinement heterostructures (GRINSCH) emit at 980 nm and have exhibited low threshold current densities, high electrical to optical power conversion efficiencies, low noise, and low temperature sensitivities [1–3]. Recently there has been much attention toward the growth and fabrication of devices containing short period superlattices of (InAs)m/(GaAs)m or (GaAs)m/(AlAs)n compositions [4–9]. Indeed the growth of inverted MODFET structures using a (GaAs)/(AlAs) superlattice barrier in place of an AlGaAs random alloy exhibited an increase in low temperature mobility due to reductions in the number of impurities and interface roughness [7]]. Can the optical properties of strained layer InGaAs/GaAs Q-W lasers also be improved by growing an (InAs)/(GaAs)4 short period superlattice as the active layer in place of the In0.2Ga0.8 As random alloy?This paper reports the fabrication of ridge waveguide lasers using a 6 period (InAs)1/(GaAs)4 superlattice active region in a GRINSCH laser structure. One period of the superlattice consists of 1 monolayer (ML) of InAs and 4 ML of GaAs. Thus the average In composition in the superlattice is 20%. This superlattice laser is an alternative to the In0.2Ga0.8As/GaAs random alloy quantum-well laser used as a pump source for Erbium Doped Fiber Amplifiers. In a single quantum well In0.2Ga0.8As/GaAs laser the active region is a thin layer (-70Å thick) sandwiched between GaAs barrier layers. The superlattice active laser reported here has 6 periods of (InAs)1/(GaAs)4 as the light emitting region of a total width comparable to a single quantum well of random alloy composition.

Largely enhanced bound-to-miniband absorption in an InGaAs multiple quantum well with short-period superlattice InAlAs/InGaAs barrier

We report for the first time, the intersubband absorption at 10.7 μm between the localized ground state and the global miniband state in an n-type InAlAs/InGaAs multiple quantum well and short-period superlattice barrier heterostructure. The use of enlarged quantum well width and the superlattice reinforced miniband structure has shown a significant enhancement in the net intersubband absorption. An integrated optical absorption strength of IA=19.5 Abs cm−1 was obtained under the Brewster’s incident angle at T=300 K, which is about five times larger than that of the conventional single bound-to-bound transition in the InAlAs/InGaAs quantum well structure.

Improved AlGaAs/GaAs double-barrier resonant tunneling structures using two-dimensional source electrons

We report the enhancement of peak-to-valley current ratios (PVCRs) of double-barrier resonant tunneling structures (DBRTSs) based on the AlGaAs/GaAs material system. The PVCRs as high as 25.4 and 18 have been obtained at 77 K for superlattice and alloy barrier structures with 0.2-μm undoped electrodes, respectively. These are the largest PVCRs to date for AlGaAs/GaAs DBRTSs. The large band bending across the undoped electrodes causes size quantization of the accumulation layer, resulting in better resonant tunneling characteristics.

Hyperabrupt current switching in [formula omitted] superlattices

We have observed extremely large hyperabrupt changes spanning many orders of magnitude in superlattice current at temperatures ranging from 55–350K. Current is in the 10pA range before threshold and rises abruptly to over 10 mA at 55K, a change of nine orders of magnitude for a voltage change of about 1.0mV. This instantaneous current change is far larger than space charge limited current increases previously reported for bulk semiconductors. A native defect located in the superlattice barriers controls the magnitude of prethreshold current. This defect has a strong temperature and electric field dependent tunneling emission to the superlattice quantum well ground states.

Time of flight of electrons and holes at 77 and 300 K in GaAs/AlAs superlattices

The carrier drift velocity versus applied electric field in GaAs/AlAs superlattices is directly determined from a photocurrent time of flight technique. The electron negative differential velocity regime is clearly evidenced and in good agreement with static current-voltage data in the case of a relatively thick barrier superlattice (7 AlAs layers). For a thinner barrier superlattice (4 AlAs layers) the low field electron mobility seems temperature dependent, and also a drastic hole mobility decrease is observed at low temperature and interpreted in terms of light hole freeze out.

Recombination processes in quantum well lasers with superlattice barriers

Spontaneous emission spectra from GaAs quantum well lasers grown by molecular beam epitaxy show that the radiative recombination rate in (AlAs)(GaAs) superlattice barriers is greater than in alloy barriers of the same average composition (x=0.25) due to reduction in effective gap by superlattice effects. Measurements of emission spectra as functions of temperature show that these radiative processes account for a significant part of the temperature variation of the threshold current and we estimate that the nonradiative lifetime in the superlattice barriers is an order of magnitude longer than in alloy barriers grown under similar conditions.

High-pressure studies of resonant tunnelling in a graded parameter superlattice and in double barrier structures of GaAs/AlAs

The authors report resonant tunnelling and thermionic emission measurements at pressures up to 9 kbar and temperatures down to 4.2 K, of p-i-n and n-i-n diodes containing a graded gap superlattice and double barrier structures, respectively, in which the wells are of GaAs and the barriers are of AlAs. The graded gap superlattice was grown by MBE and consists of five nominally 100 AA periods of GaAs/AlAs in which the 'mark-space' ratio of GaAs to AlAs is increased uniformly from 94 AA of GaAs, 6 AA of AlAs to 70 AA GaAs, 30 AA AlAs. The double barrier structures were grown by MOCVD with either approximately 70 AA wells and approximately 40 AA barriers or approximately 15 AA wells and approximately 50 AA barriers. The size of the NDR in the p-i-n structures increases with decreasing temperature as previously reported, the best samples exhibiting a ratio of maximum to minimum tunnelling current in excess of 2 at 4.2 K and 1 bar. On applying pressure, the size of the NDR at 4.2 K decreases slowly up to 5 kbar and more rapidly at higher pressures, almost vanishing by 8 kbar. A similar effect occurs in both directions of bias for the 43-72-40 AA double barrier structures, even though these have a ratio of maximum to minimum tunnelling current in excess of 10 at 4.2 K. In contrast, the 45-15-50 AA double barrier structures do not exhibit negative differential resistance, but thermionic emission measurements yield values for the barrier height, and its pressure dependence, of approximately 0.15 eV and -0.011 eV kbar-1 respectively. They take all these observations as evidence that X-like barrier states are strongly involved in the resonant tunnelling process, and that in general the pressure threshold for loss of NDR in samples grown in different ways, and with different ratios of peak to valley currents, is essentially a function only of well and barrier dimensions.

GaAs double quantum well laser diode with short-period (AlGaAs)m(GaAs)n superlattice barriers

A short-period (AlGaAs)m(GaAs)n superlattice has been applied to the barrier layers in a single and a multiple quantum well structure prepared by molecular beam epitaxy in order to improve the interface quality. With a 38 Å thin GaAs quantum well without employing aluminum, a low threshold current density of 260 A/cm2, a high characteristic temperature (T0) of 205 K, and a high differential quantum efficiency of 75% have been achieved in a double quantum well ridge waveguide laser diode emitting at 780 nm.

Broad area phase-locked superlattice barrier quantum well laser diode

The lateral coherence of an extremely broad-area laser diode is examined. The device was fabricated using molecular-beam epitaxy (MBE). For a 350- mu m-wide mesa laser, a single-lobed phase-locked narrow beam was obtained owing to the use of a high-uniformity quantum well graded-index waveguide separate confinement heterostructure (GRIN-SCH) with a short-period (AlGaAs)(GaAs) superlattice MBE-fabricated barrier layer.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Investigation of short period AlAs/GaAs superlattices as conduction-band barriers

Short period AlAs/GaAs superlattices, or pseudoalloys, are investigated as insulators in n+ GaAs–insulator–n GaAs structures, or superlattice barrier capacitors (SLBC’s), and as tunneling barriers in superlattice double barrier diodes (SLDBD’s). The energies of the lowest superlattice states in the SLBC’s and the energies of the quantum well quasibound states in the SLDBD’s are determined using thermionic emission analysis. Depending on the GaAs and AlAs layer thicknesses, the lowest states in the SLBC’s may be Γ states in the GaAs layers or X states in the AlAs layers. It is also possible for the barrier to behave as a random alloy. We observe all three cases. The SLDBD’s exhibit strong negative resistance at low temperatures and we find evidence that the superlattice barriers (SLB’s) behave as ideal Γ band structures, regardless of the layer thicknesses. Second current peaks which are attributed to resonant states in the SLB’s appear in the SLDBD I–V curves when the GaAs layers are thicker than 5 monolayers.

Effect of undoped GaAs spacers on the characteristics of GaAs-(Al,Ga)As-GaAs single barrier structures

The effects of undoped spacer layers on the electrical properties of single barrier heterostructures (both alloy and superlattice) have been investigated by measuring incremental slope resistance over the bias range −400 to +400 mV and at convenient temperature intervals between 70 and 290 K. The zero bias slope resistance, Rs(0), and the effective barrier heights increase with spacer thickness. Also, the low-temperature slope resistances, Rs(V), decrease exponentially with the magnitude of the bias, V, while the effective barrier heights, deduced from high-temperature measurements, decrease approximately linearly. This suggests that the decrease in Rs(V) with bias is due simply to the voltage-induced decrease in effective barrier height. Rs(0) varies exponentially with zero bias effective barrier height for both alloy and superlattice barriers and this is consistent with the Γ electrons dominating the current transport through the barriers. All of our Rs(V) curves are asymmetric and, using Airy function calculations, we have modeled curves similar to the experimental ones by assuming different doping levels for the two doped GaAs layers on either side of the barriers. This is possibly due to Si migration into the ‘‘undoped’’ barrier or the spacer layer closest to the substrate.

Characterization of AlAs/GaAs superlattice barriers using electrical barrier height analysis

Electrical barrier height measurements on n+-GaAs–insulator–n-GaAs structures with short-period AlAs/GaAs superlattices forming the insulator show the effective conduction-band discontinuity (ΔEC) of a superlattice barrier (SLB) to be defined by the lowest superlattice energy state. Five structures with different AlAs and GaAs SLB layer thicknesses are investigated. A SLB with GaAs layers greater than 10 monolayers is found to have a ΔEC defined by Γ-valley states in the GaAs layers, while a SLB with GaAs and AlAs layers less than 10 monolayers and with thicker AlAs layers than GaAs layers is found to have a ΔEC defined by X-valley states in the AlAs layers. The SLB with GaAs and AlAs layers less than 10 monolayers and thicker GaAs layers than AlAs layers behaves as a random alloy. Negative differential resistance is observed in the current-voltage characteristic of the sample whose barrier height is defined by Γ-valley states in the GaAs layers.

Bound-to-extended state absorption GaAs superlattice transport infrared detectors

We demonstrate an efficient 10-μm detector based on localized bound-state–to–extended-continuum-state absorption in doped GaAs/AlxGa1−xAs quantum wells designed to contain only one bound state. The voltage dependence of the responsivity is strikingly different from previous intersubband tunneling detectors containing two bound states. This results from the photoexcited electrons being transported over the top of the superlattice barriers, rather than tunneling through them.

Parametric study of AlAs/GaAs superlattice double-barrier diodes

Quantum well diodes with barriers formed by thin, short-period binary AlAs/GaAs superlattices were fabricated and found to have very high peak-to-valley current ratios. The effects of varying the AlAs and GaAs layers in the barriers are studied. The peak current density is found to decrease by orders of magnitude for monolayer increases in the AlAs layer thicknesses. Tunneling current peaks due to both resonance levels in the quantum well and resonance levels in the superlattice barriers are observed.