Heterostructure Wafers Research Articles

Andreev Reflection in the Quantum Hall Regime at an Al/InAs Junction on a Cleaved Edge.

We have fabricated a superconductor/semiconductor junction composed of Al and InAs using cleaved edge overgrowth. By exploiting the unique geometry with a thin Al/Pt/Al trilayer formed on the side surface of an in situ cleaved InAs quantum well heterostructure wafer, we achieve a superconducting critical field of ∼5 T, allowing superconductivity and quantum Hall (QH) effects to coexist down to filling factor ν = 3. Andreev reflection at zero magnetic field shows a conductance enhancement limited solely by the Fermi velocity mismatch, demonstrating a virtually barrier-free junction. Bias spectroscopy in the QH regime reveals the opening of a superconducting gap, with the reduced downstream resistance indicating electron-hole Andreev conversion. Our results, obtained in a new experimental regime characterized by a clean edge-contacted junction with a superconducting electrode narrower than the coherence length, open new avenues for both theoretical and experimental studies of the interplay between superconductivity and QH effects.

An AlGaN-GaN HEMT with p-GaN Extended Gate for Improvements on Current Dispersion and Breakdown Characteristics

This study introduces a novel p-GaN/AlGaN/GaN heterostructure wafer, implementing a unique p-type GaN gate AlGaN/GaN HEMT configuration. In this design, the p-GaN region extends toward the drain, eliminating the need for a gate electrode. This innovation significantly enhances the HEMT’s performance, with a 45.2% increase in breakdown voltage (BV) and a 17% higher threshold voltage (VTH) compared to conventional p-GaN gate HEMTs. The extended gate design redistributes the electric field, acting as a field plate to elevate the breakdown voltage. Furthermore, the proposed device, by reducing 17.4% of the saturation current without increasing the on-resistance, possibly offers improved short-circuit capability.

Control of yellow photoluminescence in AlGaN/GaN heterostructures

Photoluminescence with the peak corresponding to yellow color of the visible spectrum (so-called yellow luminescence) originates from deep levels in the GaN buffer layers of heterostructures and depends on heterostructure growth conditions. In turn deep levels affect the resistance of Ohmic contacts of microwave transistors fabricated from these heterostructures. This determines the reliability of GaN microwave transistor operation. Two types of units for control of photoluminescence with the peak in the yellow visible spectral region have been designed with the aim to control the quality of AlGaN/GaN/SiC and AlGaN/GaN/Al2O3 heterostructures. One of the units is used for fast control of yellow photoluminescence and the other for photoluminescence mapping on heterostructure wafer surfaces. Examples of photoluminescence maps for structures grown on different substrates have been given.

Electrical and optical characterization of quantum dots PbS/TiO2 based heterojunction as a SWIR detector and a proposed design of PbS/TiO2-PeLED as a SWIR to visible upconversion device

In this work we present the electrical and optical characterizations of a Short Wavelength Infrared (SWIR) photodetector, which consists of tandem FTO/TiO2/PbS colloidal quantum dot (CQD) layers. The TiO2/PbS hetero-structure can be operated either as a standalone detector or as the SWIR absorption section of a SWIR to visible up-conversion device (SWVUCD) when visible photo-emission layer is attached to infrared absorption layer such as this heterojunction. The results reveal the existence of the PbS CQD’s bandgap operating in the SWIR spectral range as well as exhibit mixed photovoltaic and photoconductive characteristics. The optical characteristics of the different grown layers were analyzed by photoluminescence and absorption methods. The current density—voltage (J–V) curve at forward bias shows a crossover effect, however, no rollover effect has been observed. The latter indicates that there is no Schottky barrier component in this structure. We have concluded that in our case the energy bandgap of the PbS CQD is 0.93 eV where a defects states level with energy of 0.83 eV in the bandgap is evident. Finally, we propose and discuss a possible architecture of such a SWIR photosensitive hetero-structure wafer bonded to a Perovskite LED in order to form an imaging SWVUCD. Such a SWVUCD, when attached to a visible CMOS sensor, can serve as a low cost SWIR based detector for eye-safe Laser Imaging Radar (LiDAR) and for SWIR night vision applications. Such an architecture is favorable over current technologies with regard to manufacturing costs, preparation complexity, power consumption, size and weight.

High quality InAlN/GaN heterostructures grown on sapphire by pulsed metal organic chemical vapor deposition

High quality InAlN/GaN heterostructures are successfully grown on the (0 0 0 1) sapphire substrate by pulsed metal organic chemical vapor deposition. The InAlN barrier layer with an indium composition of 17% is observed to be nearly lattice matched to GaN layer, and a smooth surface morphology can be obtained with root mean square roughness of 0.3 nm and without indium droplets and phase separation. The 50 mm InAlN/GaN heterostructure wafer exhibits a mobility of 1402 cm 2/V s with a sheet carrier density of 2.01×10 13 cm −2, and a low average sheet resistance of 234 Ω/cm 2 with a sheet resistance nonuniformity of 1.22%. Compared with the conventional continual growth method, PMOCVD reduces the growth temperature of the InAlN layer and the Al related prereaction in the gas phase, and consequently enhances the surface migration, and improves the crystallization quality. Furthermore, indium concentration of InAlN layer can be controlled by adjusting the pulse time ratio of TMIn to TMAl in a unit cycle, the growth temperature and pressure, as well as the InAlN layer thickness by the number of unit cycle repeats.

Deep traps in AlGaN/GaN heterostructures studied by deep level transient spectroscopy: Effect of carbon concentration in GaN buffer layers

Electrical properties, including leakage currents, threshold voltages, and deep traps, of AlGaN/GaN heterostructure wafers with different concentrations of carbon in the GaN buffer layer, have been investigated by temperature dependent current-voltage and capacitance-voltage measurements and deep level transient spectroscopy (DLTS), using Schottky barrier diodes (SBDs). It is found that (i) SBDs fabricated on the wafers with GaN buffer layers containing a low concentration of carbon (low-[C] SBD) or a high concentration of carbon (high-[C] SBD) have similar low leakage currents even at 500 K; and (ii) the low-[C] SBD exhibits a larger (negative) threshold voltage than the high-[C] SBD. Detailed DLTS measurements on the two SBDs show that (i) different trap species are seen in the two SBDs: electron traps Ax (0.9 eV), A1 (0.99 eV), and A2 (1.2 eV), and a holelike trap H1 (1.24 eV) in the low-[C] SBD; and electron traps A1, A2, and A3 (∼1.3 eV), and a holelike trap H2 (>1.3 eV) in the high-[C] SBD; (ii) for both SDBs, in the region close to GaN buffer layer, only electron traps can be detected, while in the AlGaN/GaN interface region, significant holelike traps appear; and iii) all of the deep traps show a strong dependence of the DLTS signal on filling pulse width, which indicates they are associated with extended defects, such as threading dislocations. However, the overall density of electron traps is lower in the low-[C] SBD than in the high-[C] SBD. The different traps observed in the two SBDs are thought to be mainly related to differences in microstructure (grain size and threading dislocation density) of GaN buffer layers grown at different pressures.

Fabrication and Characterization of an Enhancement-Mode Planar Resonant Tunneling Transistor

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> We report the fabrication and characterization of a planar resonant tunneling transistor (PRTT) using a GaAs/AlGaAs heterostructure wafer. Our PRTT operates as an enhancement-mode device, and shows the negative differential resistance (NDR) modulated with the gate bias in an extremely wide range (from the fully depleted dot to the dot in a single-electron transistor regime). This NDR spectrum represents a full, gate-bias-modulated evolution of the 0-D states in a quantum dot. Our data are also consistent with the ground (<formula formulatype="inline"><tex Notation="TeX"> $E_{1}$</tex></formula>) and the first excited state (<formula formulatype="inline"><tex Notation="TeX">$E_{2}$</tex></formula>) of the Fock–Darwin states. </para>

Bulk nature of sub-linear width dependent breakdown of the quantum Hall effect

We have investigated the quantum Hall effect breakdown (QHE) by using samples whose confinement potential at the sample boundary is tunable by using side gate structure. The Hall bar shaped samples are made of two types of the GaAs/AlGaAs heterostructure wafers, where the QHE breakdown current is linear or sub-linear to the sample width. In both linear and sub-linear type of samples, the value of breakdown current is scarcely affected by changing the side gate voltage, which suggests that the QHE breakdown is almost independent of the confinement potential at the 2DEG edge. We have measured the QHE breakdown with Corbino shaped samples. The critical voltage in the sub-linear type sample is much smaller (<3 kV/ m) than those in the linear type samples (≈10 kV/ m) , and the width dependence of critical voltage shows nonlinear feature. The origin of the sub-linear type QHE breakdown is not ‘edge effect’ but the properties of bulk two-dimensional electron system.

Fabrication of AlGaN/GaN Quantum Nanostructures by Methane-Based Dry Etching and Characterization of Their Electrical Properties

Attempts were made to fabricate AlGaN/GaN quantum nanostructures by dry etching, and to assess etching damage through characterization of their electrical properties. An electron-cyclotron-resonance-assisted reactive ion beam etching (ECR-RIBE) process using a CH4-based gas mixture was applied to AlGaN/GaN heterostructure wafers, and the process was characterized and optimized using in situ X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), atomic force microscopy (AFM) and photoluminescence (PL) measurements. Addition of nitrogen gas to the standard gas mixture of CH4/H2/Ar was found to be extremely effective in improving the etching characteristics, etched surface morphology and surface stoichiometry, and in preventing accumulation of Ga and Al metallic clusters. After testing the fabrication feasibility of various nanostructures, AlGaN/GaN quantum wire (QWR) structures of various widths were fabricated, and their electrical characteristics were investigated to detect etching damage. Clear Shubnikov-de Haas (SdH) oscillations were observed at 1.6 K in the QWR structures under strong magnetic fields. A nonlinear Landau plot indicated the presence of a one-dimensional confinement potential with a subband energy spacing of 1.4 meV. The measured wire conductance can be well explained by a simple theory, taking account of the sidewall depletion of 23 nm due to Fermi level pinning. No clear changes in sheet carrier concentration and mobility due to plasma damage were detected.

A Novel Thin Al 2 O 3 Gate Dielectric by ECR‐Plasma Oxidation of Al for AlGaN/GaN Insulated Gate Heterostructure Field‐Effect Transistors

A novel formation process of a thin Al2O3 gate dielectric for AlGaN/GaN insulated gate (IG) heterostructure field-effect transistors (HFETs) is presented. Al2O3 was formed in an ultrahigh vacuum environment by a process sequence consisting of deposition of a thin Al film on the surface of the AlGaN/GaN heterostructure wafer, its ECR-O2 plasma oxidation and subsequent annealing. Its composition was confirmed by in-situ X-ray photoelectron spectroscopy measurements. Fabricated ring-shaped IG-HFET test devices exhibited good gate control of two-dimensional electron gas (2DEG). Forward and reverse gate leakage currents were very much reduced in IG-HFETs, being smaller than those of Ni Schottky gate HFETs by five to seven orders of magnitude. The new gate structure seems to be extremely promising for realizing high performance AlGaN/GaN HFETs.

Relaxation of local magnetization of single-crystalline Bi 2Sr 2CaCu 2O 8 at low temperatures

The local magnetization and it's relaxation rate of a single crystalline Bi 2Sr 2CaCu 2O 8 was measured by a miniature Hall device with two-dimensional electron gas (2DEG) fabricated from a GaAs/Al x Ga 1− x As heterostructure wafer. We investigated the magnetic field and the temperature dependence of the relaxation rate at low temperatures between 100 mK and 4.2 K under the applied field ranging from 0 to 1.7 T. The obtained results show the anomalous field dependence of the relaxation rate with a large enhancement in the fields between 0.5 and 1 T. The relaxation rate shows the crossover behavior from QCC region to thermally assisted region in temperature dependence. The local relaxation theory of a critical current density based on the critical state model realized the similar anomalous field dependence of the relaxation rate, assuming a constant relaxation rate.

Thresholdlike behavior of photoluminescence in laser heterostructure wafers

Thresholdlike behavior of photoluminescence (PL) in heterostructure wafers is studied. It is shown that strictly speaking there is no PL threshold, and the thresholdlike dependence of PL on the pump power results from the combination of three factors: the PL spreading along the wafer surface, the change in the radiative fraction of electron-hole recombination, and the restricted aperture of the PL detector. The first two factors were found to be dominating in the wafers studied.

Operation of a novel negative differential conductance transistor fabricated in a strained Si quantum well

The operation of a negative differential conductance (NDC) transistor fabricated on a high-mobility Si/Si/sub 1-x/Ge/sub x/ heterostructure wafer is described. The drain characteristic of this device shows a large NDC with current peak-to-valley ratios as high as 600 (100) at T=0.4 K (T=1.3 K). The NDC can be modulated over a wide range of current levels by either of two separately-contacted gate electrodes. The device shows bistable switching behaviour in both current- and voltage-controlled circuit configurations. The novel operating principle of this transistor is described, along with its potential for future logic and memory applications.

Negative differential conductance in lateral double-barrier transistors fabricated in strained Si quantum wells

The operation of a laterally patterned negative differential conductance (NDC) device based on hot-electron effects in strained Si quantum wells is demonstrated. The device consists of a small, etch-defined dot region connected to narrow leads on either side via point-contact tunnel barriers. The entire device is fabricated on a high-mobility Si/Si1−xGex heterostructure wafer using electron-beam lithography and low-damage reactive-ion etching. At T=0.4 K (T=1.3 K), the drain characteristic of this device shows a pronounced NDC region with a peak-to-valley ratio (PVR) greater than 600 (100). The PVR is reduced with increasing temperature, with remnants of the NDC observable to 30 K. The NDC is attributed to phonon emission by hot electrons injected into the dot region.

Breakdown of the integer quantum Hall effect at high currents in GaAs/AlGaAs heterostructures

Experimental results of the breakdown of the integer quantum Hall effect (QHE) measured at Gakushuin University for butterfly-type Hall bar samples fabricated from GaAs/AlGaAs heterostructure wafers with various electron concentrations and mobilities show that the onset of the breakdown of the QHE is caused by average current density or average Hall electric field. The result supports the idea that the edge channel picture is not a realistic description of the QHE. The critical breakdown Hall fields (i = 2 and 4) and (i = 1 and 3) are proportional to where B is the magnetic field at the centre of each quantized Hall plateau. The effective g-factor in this 2D system in strong magnetic fields is evaluated as from the experimental result assuming .

Optical properties of annealed, single GaAs quantum wells: Cap doping and mask width dependence

Band edge absorption measurements are used to characterize the degree of Al-Ga intermixing (blue-shifting) and the linear optical loss of waveguides formed in five annealed and encapsulated single GaAs quantum well laser heterostructure wafers which differed only by the amount of Zn doping in the GaAs cap layer. In addition to the transmission measurements, secondary ion mass spectroscopy data was used to verify the diffusion of Zn before and after annealing. High zinc doping in the cap is observed to cause quantum well disordering below the encapsulant (Si3N4) and is attributed to impurity induced layer disordering. Moderate doping in the cap results in selective area intermixing via controlled gallium vacancy production. A stripe width dependence is also observed, which suggests a role of lateral diffusion of species which affect the intermixing. For an undoped (n−) cap, the degree of intermixing is heavily dependent on the arsenic overpressure used during the anneal and is independent of the nitride stripe width suggestive of a volumetric Fermi level dependent production of vacancies within the cap.

Influences of Interfacial Misfit Stress on Modulated Microstructures of InGaAsP / InP Heterostructures

The misfit stresses of metallorganic chemical vapor deposition grown heterostructure wafers were determined and their effects on the modulated microstructures of epilayer were investigated. Lattice mismatches were measured using {511}asymmetric and (400) symmetric x‐ray reflections. Elastic strains were obtained from the parallel mismatches and were used for determining the misfit stresses. Transmission electron microscopy results showed the presence of the modulated microstructures (spinodal decomposition) in the epilayer. It was found that the modulation periodicity varies with the lattice mismatches. To investigate this observation, the interaction elastic energy induced by misfit stress was derived and used to explain the experimental results. Good agreement was obtained between the measured and calculated modulation wavelength. Also, contributions from the composition and misfit stress to variation of modulation periodicity were separated out, and the misfit stress was found to have the larger contribution. Thus, it is concluded that the interfacial coherency and misfit stress have large influences on the microstructures of the InGaAsP epilayer, particularly on the modulation wavelength.

Very high-efficiency semiconductor wafer-bonded transparent-substrate (AlxGa1−x)0.5In0.5P/GaP light-emitting diodes

Data are presented demonstrating the operation of transparent-substrate (TS) (AlxGa1−x)0.5In0.5P/GaP light-emitting diodes (LEDs) whose efficiency exceeds that afforded by all other current LED technologies in the green to red (560–630 nm) spectral regime. A maximum luminous efficiency of 41.5 lm/W (93.2 lm/A) is realized at λ∼604 nm (20 mA, direct current). The TS (AlxGa1−x)0.5In0.5P/GaP LEDs are fabricated by selectively removing the absorbing n-type GaAs substrate of a p-n (AlxGa1−x)0.5In0.5P double heterostructure LED and wafer bonding a ‘‘transparent’’ n-GaP substrate in its place. The resulting TS (AlxGa1−x)0.5In0.5P/GaP LED lamps exhibit a twofold improvement in light output compared to absorbing-substrate (AS) (AlxGa1−x)0.5In0.5P/GaAs lamps.

Metalorganic vapor phase epitaxical growth and 1.5-μm laser fabrication using ethyldimethylindium, tertiarybutylphosphine, and tertiarybutylarsine

High-quality InGaAsP lattice matched to InP was grown by low-pressure metalorganic vapor phase epitaxy using ethyldimethylindium (EDMIn), tertiarybutylphosphine (TBP), and tertiarybutylarsine (TBA). The background carrier concentrations of undoped InP and InGaAs were as low as 1.5×1015 and 2.2×1015 cm−3, respectively. Good compositional control of In-GaAsP was also possible. The performance of the double heterostructure wafers employed as lasers as characterized by threshold current density, internal loss, and internal quantum efficiency. The threshold current density for a 300-μm cavity was as low as 2.0 kA/cm2. In addition, ridge-waveguide distributed feedback lasers were successfully fabricated. These results show that EDMIn, TBP, and TBA might be used to replace conventional sources for growing InGaAsP/InP lasers.

Substrate-epitaxial layer interface effects on AlGaAs/GaAs heterostructure device properties

Device characteristics of AlGaAs/GaAs heterostructure field effect transistors fabricated by molecular-beam epitaxial growth are related to the condition of the substrate-epitaxial layer interface. The presence of carbon on the GaAs wafer surface prior to growth has been found to produce a p-type, conducting interfacial region. We demonstrate that the concentration of carbon on the wafer surface can be significantly reduced by exposing the wafers to ultraviolet radiation. Depletion- and enhancement-mode device transfer characteristics and transconductance curves have been obtained on heterostructure wafers that were subjected to an ultraviolet-ozone surface preparation. A comparison of these results with the device properties of wafers receiving a standard cleaning procedure is presented. A model describing the interaction between the interfacial p-type region and the two-dimensional electron gas channel is also included.