P-type Quantum Research Articles

Microwave frequency characterization of undoped and p-doped quantum dot lasers

The maximum relaxation frequency of directly modulated p-type doped quantum dot (QD) lasers decreases monotonically from 4.6 to 3.6GHz with p-doping levels in the range of 20–40 holes/dot. Modulation efficiencies decrease as well with these p-type concentrations. These results are shown to originate from significantly larger internal losses in the p-doped QD devices that, despite an increased maximum ground state modal gain, causes increased gain compression and decreased differential gain for constant cavity length lasers.

Spin-dependent transport in p+-CdBxF2-x - n-CdF2 planar structures

The CV measurements and tunneling spectroscopy are used to study the ballistic transport of the spin-polarized holes by varying the value of the Rashba spin-orbit interaction (SOI) in the p-type quantum well prepared on the surface of the n-CdF2⟨Y⟩ bulk crystal. The findings of the hole conductance oscillations in the plane of the p-type quantum well that are due to the variations of the Rashba SOI are shown to be evidence of the spin transistor effect, with the amplitude of the oscillations close to e2/h.

Spin interference in silicon one-dimensional rings

We present the first findings of the spin transistor effect in the Rashba gate-controlled ring embedded in the p-type self-assembled silicon quantum well that is prepared on the n-type Si (100) surface. The coherence and phase sensitivity of the spin-dependent transport of holes are studied by varying the value of the external magnetic field and the top gate voltage that are applied perpendicularly to the plane of the double-slit ring and revealed by the Aharonov-Bohm (AB) and Aharonov-Casher (AC) conductance oscillations, respectively. Firstly, the amplitude and phase sensitivity of the 0.7.(2e2/h) feature of the hole quantum conductance staircase revealed by the quantum point contact inserted in the one of the arms of the double-slit ring are found to result from the interplay of the spontaneous spin polarization and the Rashba spin-orbit interaction (SOI). Secondly, the values of the AC conductance oscillations caused by the Rashba SOI are found to take the fractional form with both the plateaus and steps as a function of the top gate voltage.

Optical properties of p-type modulation-doped InAs quantum dot structures grown by molecular beam epitaxy

We have performed temperature-dependent studies of photoluminescence (PL) properties of p-type modulation-doped InAs/GaAs quantum dots (QDs) structures grown by molecular beam epitaxy. In temperature dependence of integrated PL intensity, full width at half maximum (FWHM) of PL spectra and its peak energy, differences between undoped and p-doped have depended on a modulation-doping level. We have observed the improvement of temperature stability of PL intensity by p-type modulation doping to InAs QDs in comparison with that of undoped. FWHM of PL spectrum of p-InAs QDs increases as modulation-doping level increases. Excessive modulation doping has caused the degrading of PL properties due to Beryllium (Be) diffusion from p-GaAs modulation-doping layer to InAs layer via undoped GaAs spacer layer. The Be diffusion into InAs region was confirmed by secondary ion mass spectroscopy.

Electronic and spin properties of hole point contacts

We have studied theoretically the effect of a tuneable lateral confinement on two-dimensional hole systems realised in III-V semiconductor heterostructures. Based on the 4 × 4 Luttinger description of the valence band, we have calculated quasi-onedimensional (quasi-1D) hole subband energies and anisotropic Landé g -factors. Confinement-induced band mixing results in the possibility to manipulate electronic and spin properties of quasi-1D hole states over a much wider range than is typically possible for confined conduction-band electrons. Our results are relevant for recent experiments where source-drain-bias spectroscopy was used to measure Zeeman splitting of holes in p-type quantum point contacts. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Strategies to Improve Optical Gain and Waveguide Loss in Strain-Compensated SiGe Quantum Cascade Mid-Infrared Emitters

Strain-compensated p-type SiGe quantum cascade (QC) structures are promising candidates to realize an efficient Si-based light source. In this paper, we present the strategies to improve the optical gain and to minimize the waveguide loss to develop a SiGe QC laser for mid-infrared wavelength

Ballistic transport in induced one-dimensional hole systems

The authors have fabricated and studied a ballistic one-dimensional p-type quantum wire using an undoped AlGaAs∕GaAs heterostructure. The absence of modulation doping eliminates remote ionized impurity scattering and allows high mobilities to be achieved over a wide range of hole densities and, in particular, at very low densities where carrier-carrier interactions are strongest. The device exhibits clear quantized conductance plateaus with highly stable gate characteristics. These devices provide opportunities for studying spin-orbit coupling and interaction effects in mesoscopic hole systems in the strong interaction regime where rs>10.

Study on the p-type QWIP-LED device

A p-type quantum well infrared photodetector (QWIP) integrated with a light-emitting diode (LED) (named QWIP-LED) was fabricated and studied. The infrared photo-response spectrum was obtained from the device resistance variation and the near-infrared photo-emission intensity variation. A good agreement between these two spectra was observed, which demonstrates that the long-wavelength infrared radiation around 7.5 μm has been transferred to the near-infrared light at 0.8 μm by the photo-electronic process in the QWIP-LED structure. Moreover, the experimentally observed infrared response wavelength is in good agreement with the theoretical calculation value of 7.7 μm. The results on the upconversion of the infrared radiation will be very useful for the new infrared focal plane array technology.

Lateral conduction quantum dot infrared photodetectors using photoionization of holes inInAs quantum dots

We report on mid-infrared photoconductivity of Be-doped p-type lateral conduction quantum dotinfrared photodetectors and electronic states of the valence band in InAs self-assembled quantumdots. We have observed a broad photocurrent signal in a photon energy range of 100–400 meV(λ∼3–10 µm) due to bound-to-continuum intersublevel absorption of normal incidence radiation in thevalence band of InAs self-assembled quantum dots, showing a peak sensitivity aroundhν = 190 meV(λ∼6.5 µm). The observed responsivity was as high as16 A W−1 atT = 10 K. Furthermore, the activation energy from ground state in the quantum dot to the valenceband of GaAs was estimated to be around 100 meV.

Resonant Intrinsic Spin Hall Effect inp-Type GaAs Quantum Well Structure

We study intrinsic spin Hall effect in p-type GaAs quantum well structure described by Luttinger Hamiltonian and a Rashba spin-orbit coupling arising from the structural inversion symmetry breaking. The Rashba term induces an energy level crossing in the lowest heavy hole subband, which gives rise to a resonant spin Hall conductance. The resonance may be used to identify the intrinsic spin Hall effect in experiments.

Spin interference in silicon one-dimensional rings

We present the first findings of the spin transistor effect caused by the Rashba gate-controlled ring embedded in the p-type self-assembled silicon quantum well that is prepared on the Si (1 0 0) surface using the planar diffusion CVD technology. The coherence and phase sensitivity of spin-dependent transport of holes are studied by varying the value of external magnetic field and gate voltage that are perpendicular to the plane of the double-slit ring. Firstly, quantum scatterers connected to two one-dimensional leads and quantum point contact inserted in the one of the arms of the double-slit ring are shown to define the amplitude and the phase of the Aharonov–Bohm and the Aharonov–Casher conductance oscillations. Secondly, the amplitude and phase sensitivity of the 0.7 (2e 2/h) feature of hole quantum conductance staircase revealed by the quantum point contact inserted are found to result from the interplay of spontaneous spin polarization and Rashba spin-orbit interaction.

Superconductivity in silicon nanostructures

We present the findings of single-hole and pair-hole tunneling into the negative-U centres at the quantum well—δ-barrier interfaces that seems to give rise to the superconductivity observed in the silicon nanostructures prepared by short time diffusion of boron on the n-type Si(100) surface. These Si-based nanostructures represent the p-type ultra-narrow self-assembled silicon quantum wells, 2nm, confined by the δ-barriers heavily doped with boron, 3nm. The EPR and the thermo-emf studies show that the δ-barriers appear to consist of the trigonal dipole centres, B+–B−, which are caused by the negative-U reconstruction of the shallow boron acceptors, 2B0→B++B−. Using the CV and thermo-emf techniques, the transport of two-dimensional holes inside SQW is demonstrated to be accompanied by single-hole tunneling through these negative-U centres that results in the superconductivity of the δ-barriers, which seems to be in frameworks of the mechanism suggested by E. Simanek and C.S. Ting (Sol. St. Commun. 32 (1979) 731; Phys. Rev. Lett. 45 (1980) 1213). The values of the correlation gaps obtained from these measurements are in a good agreement with the data derived from the temperature and magnetic field dependencies of the magnetic susceptibility, which reveal a strong diamagnetism and additionally identify the superconductor gap value.

The effect of distribution of holes in the momentum-space on the photoresponse of p-type quantum well infrared photodetector

The electronic structure and photoresponse of p-type quantum well infrared photodetector (QWIP) are calculated by self-consistent method. The effect of distribution of holes in the momentum-space on the photoresponse of the device is studied. The calculation results show that the contribution of holes to the photocurrent depends on the distribution of holes in the momentum-space, so the density of holes and the temperature influence the photoresponse of the p-type QWIP. The theoretical results agree well in the change of photoresponse of device with doping concentration and temperature in the experiment.

Intersubband absorption of strain-compensated Si1−xGex valence-band quantum wells with 0.7⩽x⩽0.85

Strain-compensated, p-type SiGe quantum wells with a high Ge concentration of up to 85% have been grown on commercially available Si0.5Ge0.5 pseudosubstrates by molecular-beam epitaxy. Structural investigations by transmission electron microscopy and high-resolution x-ray reflection and diffraction showed that at a growth temperature around T=300°C, samples in excellent compliance with the design parameters, comparatively sharp interfaces, and negligible increase of growth-induced surface roughness can be grown. Comparison of polarization-dependent intersubband absorption measurements with simulated intersubband absorption spectra shows that for the quantum wells investigated in this work, the hole eigenstates, their in-plane dispersion, and the polarization-dependent intersubband transition matrix elements are accurately described by a strain-dependent, six-band k∙p Luttinger-Kohn Hamiltonian in which only one fitting parameter—the intersubband transition linewidth—is used.

The evolution of electroluminescence in Ge quantum-dot diodes with the fold number

The electroluminescence of the light-emitting diodes with five-, ten and 30-fold p-type Ge quantum dots grown on n+ Si substrates is studied. The enhanced integral electroluminescence intensity and blueshift of the 30-fold one at high temperature (>200K) act contrary to those in five- and ten-fold ones. It is attributed to the emission in the higher-fold quantum dots enabled by the injected electrons diffusing the farther at the higher temperature. Transmission electron microscopy shows that the size of the Ge quantum dots and the Si component in them, both increase with increasing the fold number. Due to the strain-induced intermixing at the high-fold quantum dots, those dots hence have large band gap and result in the intensity increment and blueshift at the high temperature.

Formation of a self-consistent double quantum well in a wide p-type quantum well

The process of formation of self-consistent double quantum wells (DQWs) in a wide p-type quantum well in the presence of uniaxial strain is investigated. A feature of p-type systems is the structure of the valence band, which consists of two branches of energy dispersion—light and heavy holes. It is shown that this feature leads to significant splitting of the subbands of symmetric and antisymmetric states, as a result of which it is difficult to form states of the DQW with a vanishingly small tunneling gap; a uniaxial strain, by lifting the degeneracy of the band, suppresses this property, so that the two ground subbands of the size quantization of the DQW remain degenerate to high energies.

Transport and absorption in strain-compensated Si/Si 1−xGe x multiple quantum well and cascade structures deposited on Si 0.5Ge 0.5 pseudosubstrates

Various strain-compensated Si/SiGe quantum well structures with high Ge concentrations in the SiGe wells (70–85% Ge) have been successfully deposited by very low-temperature molecular beam epitaxy on thick strain-relaxed Si 0.5Ge 0.5 buffer layers. The samples have been subject to intensive and careful characterization of their structural, electronic and optical properties. X-ray diffraction and reflectivity measurements using synchrotron radiation allowed accurate determination of layer thicknesses and composition of the quantum well structures. The r.m.s. roughness at Si/SiGe interfaces is less than 0.4 nm I– V characteristics of resonant tunneling devices reveal strong negative differential resistance peaks for heavy hole tunneling but no trace for tunneling via light hole states. Two-dimensional hole gases with a mobility of up to 0.49 cm 2/Vs in 7 nm wide, modulation-doped Si 0.8Ge 0.2 quantum wells with Si barriers and relaxed Si 0.5Ge 0.5 spacer layers underline the high structural quality. The dependence of hole mobility on well width can be explained by an interface roughness of 0.4 nm, in accordance with the X-ray reflectivity measurements, and the associated fluctuation of the strain at the interfaces. Using the structural parameters determined by X-ray analysis as input parameters, the intersubband absorption spectra of strain-compensated quantum well structures have been calculated by the 6 band k· p method. The theoretical predicted spectra are in excellent agreement with those obtained by experiments. Our comprehensive study results in a set of very congruent data enabling to custom design complex Si/SiGe structures such as p-type quantum cascade emitters.

Spin depolarization in quantum wires polarized spontaneously in zero magnetic field

The conditions for a spontaneous spin polarization in a quantum wire positioned in a zero magnetic field are analyzed under weak population of one-dimensional subbands that gives rise to the efficient quenching of the kinetic energy by the exchange energy of carriers. The critical linear concentration of carriers above which the quasi one-dimensional gas undergoes a complete spin depolarization is determined by the Hartree-Fock approximation. The dependence of the critical linear concentration on the concentration of carriers is defined to reveal the interplay of the spin depolarization with the evolution of the 0.7 (2e2/h) feature in the quantum conductance staircase from the e2/h to 3/2 (e2/h) values. This dependence is used to study the effect of the hole concentration on the 0.7 (2e2/h) feature in the quantum conductance staircase of the quantum wire prepared inside the p-type silicon quantum well using the split-gate technique. The 1D channel is demonstrated to be spin-polarized at the linear concentration of holes lower than the critical linear concentration, because the 0.7 (2e2/h) feature is close to the value of 0.5 (2e2/h) that indicates the spin degeneracy lifting for the first step of the quantum conductance staircase. The 0.7 (2e2/h) feature is found to take however its normal magnitude when the linear concentration of holes attains the critical value corresponding to the spin depolarization. The variations in the height of the 0.7 (2e2/h) feature observed in the hole quantum conductance staircase that is revealed by the p-type silicon quantum wire seem to be related to the evidences of the quantum conductance staircase obtained by varying the concentration of electrons in the 1D channel prepared inside the GaAs-AlGaAs heterojunction.

Effect of cluster-type on the ferromagnetism of a GaMnN quantum well

The investigation of magnetic properties of a p-type GaMnN quantum well is presented under the theory based on the hole-mediated ferromagnetism. This study is applicable to the diluted magnetic semiconductor quantum well having both randomly distributed Mn ions and quasi-two-dimensional Mn x N y clusters. In addition to the spin-exchange interaction between a free carrier and a randomly distributed Mn ion, the interaction between a carrier and a cluster is considered. This interaction occurs due to the penetration of carrier wavefunction into the cluster. The critical temperature of the second order phase transition between ferromagnetic and paramagnetic phases is obtained from the free energy under the assumption that holes occupy the lowest sub-band only. The magnetic phase of this system is always ferromagnetic and the Curie temperature is above room temperature regardless of the types of magnetic phase of the clusters. The Curie temperature of the GaMnN quantum well having clusters depends on the composition rate of clusters.

A Double-Barrier-Emitter Triangular-Barrier Optoelectronic Switch

In this study, a triangular-barrier and a double-barrier structure are combined to form a double-barrier-emitter triangular-barrier optoelectronic switch (DTOS). In the structure center of the triangular barrier, a p-type delta-doped quantum well is inserted to enhance the hole confinement. Owing to the resonant tunneling through the double-barrier structure and avalanche multiplication in the reverse-biased junction, N-shaped and S-shaped negative-differential-resistance (NDR) phenomena occur in the current-voltage (I--V) characteristics under normal and reverse operation modes, respectively. The NDR characteristics show variations from dark to illumination conditions. Temperature effects on the NDRs of the DTOS are also obvious. The illumination and temperature influences on the device characteristics are investigated in this paper.