Modulation-doped Heterostructures Research Articles

Effect of the Inter-Subband Scattering in Modulation-Doped AlxGa1-xN/GaN Heterostructures

AbstractMagnetotransport properties of Al0.22Ga0.78N/GaN modulation-doped heterostructures have been studied at low temperatures and high magnetic fields. The inter-subband scattering of the two-dimensional electron gas was observed. The inter-subband scattering is very weak and depends weakly on temperature when temperature is between 1.3 K and 10 K and becomes stronger with increasing temperature when temperature is higher than 10 K. The strain relaxation of the Al0.22Ga0.78N layer influences the inter-subband scattering. It is suggested that the inter-subband scattering is dominant by the elastic scattering when temperature is lower than 10 K, and changes to be dominant by the inelastic scattering of the acoustic phonons when temperature is higher than 10 K.

Electron cloud effect on current injection across a Schottky contact

The electron cloud that is formed in the narrow gap material in a modulation-doped heterostructure affects the Schottky contact made to the wide gap material. It also influences absorption and collection of the optically generated carriers. Photocurrent spectra, current–voltage, and current–temperature measurements show that the increase in electron cloud density decreases dark current flow while increasing photoresponsivity. We propose that the Coulombic interaction between the confined electron cloud and the emitted electrons from metal to the wide gap material increases the barrier height. The electric field in the direction of growth due to modulation doping accounts for the increase in photoelectron collection efficiency. Implementation of this effect increases efficiency of photodetectors while, simultaneously, reducing the noise due to dark current.

Origin of current instability in GaAs/AlGaAs heterostructures

We propose a mechanism to explain the electric instability often observed in modulation-doped heterostructures GaAs/AlGaAs when current is passed along the heterostructure layers. The instability is caused by hot electron transport in AlGaAs layer that is not only heavily doped, but also strongly compensated due to the presence of DX-centers. This layer contains a large-scale random potential of significant magnitude, which strongly affects electron transport. The heating of electrons in the percolation cluster net and electron transfer from the cluster into the random potential wells result in the appearance of latent negative differential conductivity causing the current instability. When the instability gives rise to the formation of a high electric field domain, one of the domain walls blocks the current flow through the two-dimensional electron gas. Experimental results supporting this mechanism are given.

Negative differential conductance in planar one-dimensional/zero-dimensional/one-dimensional GaAs/AlGaAs structures

By using high-resolution electron beam lithography and wet etching, we have fabricated semiconducting islands between quantum wires from GaAs/AlGaAs modulation-doped heterostructures. The zero-dimensional islands were realized by displacing a 180 nm section at the center of a 1 μm long and 180 nm wide straight wire in well-controlled steps perpendicular to the wire direction. The narrow connections between the island and the wires introduce potential barriers. Finite bias voltages applied between the source and the drain of the electron waveguide structures permit to observe negative differential conductance, which is interpreted in terms of resonant tunneling of electrons through the barriers. Furthermore, we observe Coulomb blockade oscillations in the differential conductance of the structures.

An electrically and optically gate-controlled Schottky/2DEG varactor

A novel gate-controlled varactor is reported. The three-terminal varactor is a modulation-doped heterostructure of AlGaAs/GaAs with two Schottky contacts directly made to a two-dimensional electron gas (2DEG). The third, gate, contact is formed from highly doped n/sup +/ GaAs material to allow an open optical window that can be used for optical gating and mixing. Structure capacitance is less than 1 pF and a change of more than 30% from the zero bias capacitance is observed with the applied gate voltage. The capacitance also increases proportionally with applied light and inversely with the terminal voltage.

Diffuse reflectance spectroscopy for in situ process monitoring and control during molecular beam epitaxy growth of InGaAs/AlGaAs pseudomorphic high electron mobility transistors

We report the use of diffuse reflectance spectroscopy for active, closed-loop control of substrate temperature during the growth of a modulation doped heterostructure. Measurement and control of substrate temperature is a common difficulty for molecular beam epitaxy (MBE), as well as other semiconductor deposition techniques. To examine the effect of the temperature lag experienced during conventional MBE, we have grown identical pairs of GaAs/InGaAs/AlGaAs pseudomorphic high electron mobility transistors (pHEMTs). For one pHEMT in each pair, the input signal for substrate temperature control was the standard thermocouple (TC); for the other, it was diffuse reflectance spectroscopy (DRS). Under TC control, an overshoot of up to 70 °C was observed during the temperature upramp following the lower-temperature deposition of the InGaAs layer. This overshoot was eliminated under DRS control. Temperature errors can lead to heterolayer changes such as segregation, desorption, and changes in dopant activation.

Decrease of channel conductivity with increasing sheet electron concentration in modulation-doped heterostructures

The great decrease of electron mobility with increasing sheet electron concentration nS>5×1015 m−2 in modulation-doped Al0.25Ga0.75As/GaAs/Al0.25Ga0.75As and Al0.25Ga0.75As/In0.19Ga0.81As/GaAs quantum wells is observed experimentally. At nS>1016 m−2 a conductivity decreases with increasing the sheet electron concentration. The calculations of electron mobility limited by polar optical (PO) phonon scattering show that the great increase of electron intrasubband scattering by emission of PO phonons when nS exceeds 1015 m−2 is responsible for the mobility and conductivity decrease. When nS changes in the range of 1015−1017 m−2, the alternate increase and decrease of channel conductivity is observed.

Room temperature far infrared (8/spl sim/10 μm) photodetectors using self-assembled InAs quantum dots with high detectivity

Room temperature operation of far-infrared detectors made of self-assembled quantum dots embedded in the channel region of modulation-doped heterostructures is demonstrated. At room temperature, the detector shows a low dark current ranging in the nano-amperes at a bias voltage of 10 V. After the optimization of the separation between the quantum dot region and the 2DEG, a peak responsivity of 5.3 A/W is obtained at 9.0 /spl mu/m. The high detectivities of 6/spl times/10/sup 8/ and 5/spl times/10/sup 10/ cmHz/sup 1/2//W are obtained at room temperature and 80 K, respectively.

P-type Ge channel modulation doped heterostructures with very high room-temperature mobilities

p-type Si/Si0.3Ge0.7/Ge/Si0.3Ge0.7 modulation-doped (MOD) heterostructures with very high room-temperature mobility (1700cm2/Vs) have been successfully obtained by improving low-temperature (LT) buffer layers as pseudo SiGe substrates. The Ge channel MOD structures were fabricated on top of two kinds of LT buffer layers (Si0.3Ge0.7), which both had very thin layers (0.5–1 μm) and excellent properties such as low threading dislocation density (TDD) (1×105cm−2), small surface roughness (1–3 nm), and almost total relaxation (>95%), which were far superior to those of compositionally graded buffer layers (TDD: 107cm−2, surface roughness: >10 nm). The effect of the Ge/SiGe interface roughness in both kinds of LT buffer layers on the mobility was studied.

Resistively coupled single-electron transistor using tunnel gate resistor

A resistively coupled single-electron transistor (R-SET) was fabricated using a modulation-doped heterostructure and metal Schottky gates, and measured at low temperature. Currents of R-SET with tunnel gate resistor were calculated using the orthodox theory. It is shown that the R-SET with tunnel gate resistor has quite similar properties to the originally proposed R-SET.

Oscillatory cyclotron resonance effective mass near ν=1 and 2 in modulation-doped ZnSe/Zn1−x−yCdxMnySe

The fabrication by molecular-beam epitaxy of modulation-doped magnetic semiconductor heterostructures results in the creation of novel “magnetic” two-dimensional electron gases (2DEGs) with unusual transport properties. We report on the measurements of cyclotron resonance (CR) and effective mass in magnetic 2DEGs formed in modulation-doped ZnSe/Zn1−x−yCdxMnySe single quantum wells. Far-infrared absorption measurements are carried out at a nominal temperature of 4.2 K and in fields up to 30 T on these magnetic samples, as well as on corresponding nonmagnetic samples (ZnSe/Zn1−xCdxSe single quantum wells). The samples have carrier densities ranging from 1 to 4×1011 cm−2 and mobilities as high as 42 000 cm2/V s for the nonmagnetic samples. For the magnetic samples, the magnetic field dependence of the CR frequency shows marked oscillatory deviations from linearity in the vicinity of integer filling factors ν=2 and ν=1. For instance, compared to the nonmagnetic samples, the amplitude of the oscillations in the effective mass of the magnetic samples changes by 15% at half-filling factor. This is significantly larger than similar oscillatory effective mass behavior observed in standard semiconductor 2DEGS (e.g., GaAs/GaAlAs), suggesting a qualitatively different physical origin for this phenomenon in these magnetic systems.

Quantum Hall effect in the highly spin-polarized electron system

Transport study of high electron mobility modulation-doped heterostructures of CdMnTe/CdMgTe : I shows that the magnetic ions and the associated giant-spin splitting leave the quantization precision of the Hall plateaus unchanged. Positive magnetoresistance in weak magnetic fields is dominated by a spin effect. Properties of the observed two-dimensional metallic phase support its electron–electron interaction origin.

Electrical properties of InAlAs/InAsxP1-x/InP composite-channel modulation-doped structures grown by solid source molecular beam epitaxy

We report on the electrical characteristics of the two-dimensional electron gas (2DEG) formed in an InAlAs/InAsxP1-x/InP pseudomorphic composite-channel modulation-doped (MD) structure grown by solid source (arsenic and phosphorus) molecular beam epitaxy (SSMBE). The As composition, x, of strained InAsxP1-x was determined by x-ray diffraction analysis of InP/InAsxP1-x/InP multi-quantum wells (MQWs) with compositions of x=0.14 to x=0.72. As the As composition increases, the room temperature sheet resistance of InAlAs/InAsxP1-x/InP composite-channel MD structures grown over a range of As compositions decreased from 510 to 250 Ω/cm2, resulting from the greater 2DEG confinement and lower electron effective mass in the InAsxP1-x channel as x increases. The influence of growth conditions and epitaxial layer designs on the 2DEG mobility and concentration were investigated using 300 K and 77 K Hall measurements. As the exposure time of the As4 flux on the growth front of InAsxP1-x increased during growth interruptions, the 2DEG mobility, in particular the 77K mobility, was considerably degraded due to increased roughness at the InAlAs/InAsxP1-x interface. For the InAlAs/InAs0.6P0.4/InP composite-channel MD structure with a spacer thickness of 8 nm, the room temperature 2DEG mobility and density were 7200 cm2/Vs and 2.5 × 1012 cm−2, respectively. These results show the great potential of the InAlAs/InAsxP1-x/InP pseudomorphic composite-channel MD heterostructure for high frequency, power device applications.

Spin-splitting transport in In 0.75Ga 0.25As/In 0.75Al 0.25As quantum wire field-effect-transistor

We have made quantum wire field-effect-transistor (QWR-FET) in normal-type In 0.75Ga 0.25As/In 0.75Al 0.25As modulation-doped heterostructure and investigated spin-splitting properties by low-temperature magnetoresistance (MR) measurements. Maximum value of zero-field spin–orbit coupling constant, α zero, obtained here is 78 (×10 −12 eVm). We also confirmed an in-plane anisotropy of α zero, that is, different α zeros in between 〈 1 ̄ 1 0〉 and 〈1 1 0〉 directed samples and a reduction of α zero by applying negative gate biases. In MR measurements, we also observed conductance fluctuations superimposing on the beat oscillation in the sample with widths less than 2 μm.

Bound-to-continuum intersubband photoconductivity of self-assembled InAs quantum dots in modulation-doped heterostructures

We have designed and fabricated a quantum dot infrared photodetector which utilizes the lateral transport of photoexcited carriers in the modulation-doped AlGaAs/GaAs two-dimensional (2D) channels. A broad photocurrent signal has been observed in the photon energy range of 100–300 meV due to the bound-to-continuum intersubband absorption of normal incidence radiation in the self-assembled InAs quantum dots. A peak responsivity was as high as 4.7 A/W. The high responsivity is realized mainly by a high mobility and a long lifetime of photoexcited carriers in the modulation-doped 2D channels. Furthermore, it is found that the observed photosensitivity survives up to 190 K.

Quantum dots fabricated by selective area MOVPE and their application to single electron devices

A novel method of formation of uniform GaAs quantum dot (QD) structures, using selective area metalorganic vapour phase epitaxy (SA-MOVPE), and their application to single electron transistors (SETs) are demonstrated. The SiN x -coated substrates having a wire-like opening with three prominences are used. The wire-like opening is aligned in the [110] direction, which corresponds to channel region of SET. AlGaAs/GaAs modulation-doped heterostructures are grown on these substrates. Due to three prominences on the wire, the quasi-one-dimensional electron gas (Q-1DEG) channel, having a periodic variation in its width, are naturally formed. This leads to the formation of a quantum dot near the central prominence and two tunneling barriers beside the dot, which are connected to quantum wires.I D−V G characteristics under constant source-drain bias condition show clear conductance oscilations near the pinch-off, and oscillations are observed up to 65 K.I D−V DS characteristics measured at 2·1 K show clear Coulomb blockade. The results indicate the formation of SET by SA-MOVPE. Using similar method, resistance-load single electron inverter circuit is also fabricated.

Theoretical transport studies of p-type GaN/AlGaN modulation-doped heterostructures

We have calculated hole transfer and low-temperature mobilities in p-type GaN/AlGaN modulation-doped heterostructures. Although substantial p-type conduction is difficult to achieve in bulk nitrides, the strain-induced polarization field can greatly enhance the transfer of holes from relatively deep Mg acceptors in the AlGaN barrier into the GaN well. The calculations predict formation of a two-dimensional hole gas with densities greater than 1012 cm−2 and with low-temperature mobilities in excess of 104 cm2/V s.

Temperature-dependent Hall scattering factor and drift mobility in remotely doped Si:B/SiGe/Si heterostructures

Hall-and-Strip measurements on modulation-doped SiGe heterostructures and combined Hall and capacitance–voltage measurements on metal-oxide-semiconductor (MOS)-gated enhancement mode structures have been used to deduce Hall scattering factors, rH, in the Si1−xGex two-dimensional hole gas. At 300 K, rH was found to be equal to 0.4 for x=0.2 and x=0.3. Knowing rH, it is possible to calculate the 300 K drift mobilities in the modulation-doped structures which are found to be 400 cm2 V−1 s−1 at a carrier density of 3.3×1011 cm−2 for x=0.2 and 300 cm2 V−1 s−1 at 6.3×1011 cm−2 for x=0.3, factors of between 1.5 and 2.0 greater than a Si pMOS control.

Charging of deep-level centers and negative persistent photoconductivity in modulationdoped AlGaAs/GaAs heterostructures

The relaxation kinetics of persistent photoconductivity in AlGaAs/GaAs modulation-doped heterostructures due to charging of EL2-and DX-centers is investigated over a wide range of temperatures and excitation photon energies. The light-induced charging of these deep centers was found to lead to accumulation of positive and negative localized charges, which give rise to positive and negative persistent photoconductivities, respectively. These positive and negative charges are accumulated in different parts of the heterostructure. Their different characteristic times, and the different temperature dependences of these times, result in nonmonotonic time and temperature dependences of the persistent photoconductivity. Charging of EL2-centers in the GaAs buffer layer leads to negative persistent conductivity in the temperature range 180–300 K, while the negative photoconductivity observed at the temperatures below 180 K is caused by excited states of DX-centers in the n+-AlGaAs.

Analysis of quantum lifetime behaviour in modulation-doped n-channel structures

Shubnikov-de Haas measurements between 0.06 and 1.6 K have been performed on two modulation-doped n-type heterostructures, and analysed to extract the quantum lifetime. Use of the conventional Dingle formula resulted in deviations from the expected theoretical behaviour above . The corresponding quantum lifetime appeared to increase with temperature in the same range. The data were then re-analysed using a modified expression, in which the thermal damping term was neglected. This gave plots with the correct characteristics, and a quantum lifetime which was approximately constant with temperature, as expected for ionized impurity scattering in a degenerate electron gas. A decrease in the quantum lifetime was observed above , and this is attributed to increased small-angle scattering due to acoustic phonons.