M. Edirisooriya

Radiative and interfacial recombination in CdTe heterostructures

Double heterostructures (DH) were produced consisting of a CdTe film between two wide band gap barriers of CdMgTe alloy. A combined method was developed to quantify radiative and non-radiative recombination rates by examining the dependence of photoluminescence (PL) on both excitation intensity and time. The measured PL characteristics, and the interface state density extracted by modeling, indicate that the radiative efficiency of CdMgTe/CdTe DHs is comparable to that of AlGaAs/GaAs DHs, with interface state densities in the low 1010 cm−2 and carrier lifetimes as long as 240 ns. The radiative recombination coefficient of CdTe is found to be near 10−10 cm3s−1. CdTe film growth on bulk CdTe substrates resulted in a homoepitaxial interface layer with a high non-radiative recombination rate.

InAs hole inversion and bandgap interface state density of 2 x 10(11) cm(-2) eV(-1) at HfO2/InAs interfaces

High-k/InAs interfaces have been manufactured using InAs surface oxygen termination and low temperature atomic layer deposition of HfO2. Capacitance–voltage (C–V) curves revert to essentially classical shape revealing mobile carrier response in accumulation and depletion, hole inversion is observed, and predicted minority carrier response frequency in the hundred kHz range is experimentally confirmed; reference samples using conventional techniques show a trap dominated capacitance response. C–V curves have been fitted using advanced models including nonparabolicity and Fermi-Dirac distribution. For an equivalent oxide thickness of 1.3 nm, an interface state density Dit = 2.2 × 1011 cm−2 eV−1 has been obtained throughout the InAs bandgap.

Impact of extended defects on recombination in CdTe heterostructures grown by molecular beam epitaxy

Heterostructures with CdTe and CdTe1-xSex (x ∼ 0.01) absorbers between two wider-band-gap Cd1-xMgxTe barriers (x ∼ 0.25–0.3) were grown by molecular beam epitaxy to study carrier generation and recombination in bulk materials with passivated interfaces. Using a combination of confocal photoluminescence (PL), time-resolved PL, and low-temperature PL emission spectroscopy, two extended defect types were identified and the impact of these defects on charge-carrier recombination was analyzed. The dominant defects identified by confocal PL were dislocations in samples grown on (211)B CdTe substrates and crystallographic twinning-related defects in samples on (100)-oriented InSb substrates. Low-temperature PL shows that twin-related defects have a zero-phonon energy of 1.460 eV and a Huang-Rhys factor of 1.50, while dislocation-dominated samples have a 1.473-eV zero-phonon energy and a Huang-Rhys factor of 1.22. The charge carrier diffusion length near both types of defects is ∼6 μm, suggesting that recombinati...

Field-Effect Mobility of InAs Surface Channel nMOSFET With Low

Frequency (100 Hz ≤ f ≤ 1 MHz) and temperature (-50 ≤ T 20 °C) characteristics of low interface state density D_it high-κ gate-stacks on n-InAs have been investigated. Capacitance-voltage (C-V) curves exhibit typical accumulation/depletion/inversion behavior with midgap D_it of 2 × 10¹¹ and 4 × 10¹¹ cm^-2 eV^-1 at -50 °C and 20 °C, respectively. Asymmetry of low-frequency C-V curves and C-T dependence for negative voltage showing a sharp transition of ≅-20 dB/decade between low- and high-frequency behavior indicate surface inversion. An inversion carrier activation energy and an InAs hole lifetime of 0.32 eV and 2 ns have been extracted, respectively. Surface channel nMOSFETs with gate length L_g = 1 μm, channel thickness = 10 nm, and equivalent oxide thickness (EOT) 1 ≤ EOT ≤ 1.6 nm have been fabricated. For EOT = 1 nm, a subthreshold swing S = 65 mV/decade, transconductance g_m = 1.6 mS/μm, and ON-current I_ON = 426 μA/μm at an OFF-current I_OFF = 100 nA/μm (supply voltage V_dd = 0.5 V) have been measured. Peak electron field-effect mobilities of 6000-7000 cm²/Vs at sheet electron densities of 2-3 × 10¹² cm^-2 were obtained for EOT as small as 1 nm.

D_{\rm it}

One of the major challenges of high mobility complementary metal-oxide-semiconductor (CMOS) circuits is to meet off-current requirements of <100 pA/μm for low stand-by power (LSTP) operation due to the small bandgap (≤0.5 eV) of the channel material (bandgap limit). In this work, we present experimental proof that the bandgap limit can be overcome at nanometer dimensions leveraging the phenomenon of steady state deep depletion (SSDD). The occurrence of SSDD is investigated using high-k capacitors with 5 and 10 nm InAs channel on a n- or p-type doped lattice matched wide bandgap AlAsSb layer. Absence of charge carriers at the off-state band edge is observed for 5 nm InAs channel layers demonstrating occurrence of SSDD and lifting of the off-state bandgap limit providing a path to meet LSTP requirements for future high mobility CMOS.

Scaled Gate-Stack

Record setting III-V MOSFETs are reported. For the first time performance better than state-of-the-art HEMTs is demonstrated. For a MOSFET with 10 nm unstrained InAs surface channel and L_g = 130 nm operating at 0.5 V, on-current as high as I_on = 601 μA/μm (at fixed I_off = 100 nA/μm) is achieved. This record performance is enabled by g_{m, ext} = 2.72 mS/μm and S = 85 mV/dec, DIBL = 40 mV/V, resulting from breakthroughs in epitaxy and III-V/dielectric interface engineering. Measured mobility is 7100 cm²/V.s at n_s = 6.7×10¹² cm^-2. Device simulations further elucidate the performance potential of III-V N-MOSFETs.

Lifting the off-state bandgap limit in InAs channel metal-oxide-semiconductor heterostructures of nanometer dimensions

CdSeTe/CdMgTe double heterostructures were produced with both n-type and unintentionally doped absorber layers. Measurements of the dependence of photoluminescence intensity on excitation intensity were carried out, as well as measurements of time-resolved photoluminescence decay after an excitation pulse. It was found that decay times under very low photon injection conditions are dominated by a non-radiative Shockley-Read-Hall process described using a recombination center with an asymmetric capture cross section, where the cross section for holes is larger than that for electrons. As a result of the asymmetry, the center effectively extends photoluminescence decay by a hole trapping phenomenon. A reduction in electron capture cross section appeared at doping densities over 1016cm−3. An analysis of the excitation intensity dependence of room temperature photoluminescence revealed a strong relationship with doping concentration. This allows estimates of the carrier concentration to be made through a non-...

InAs N-MOSFETs with record performance of I on = 600 μA/μm at I off = 100 nA/μm (V d = 0.5 V)

An experimental study of the hole effective mass was conducted in a series of five remotely doped InSb quantum wells under biaxial compressive strain. From cyclotron resonance measurements at 4.2 K, an increase in hole effective mass was observed with increasing hole density in otherwise similar structures from 0.045me at 2.1×1011 cm−2 to 0.083me at 5.1×1011 cm−2. The smallest effective mass (0.017me) was observed in the quantum well with the largest compressive strain (1.05%) and narrowest well width (7 nm). Our measurements are in qualitative agreement with theoretical expectations.

Factors influencing photoluminescence and photocarrier lifetime in CdSeTe/CdMgTe double heterostructures

Time-resolved and time integrated photoluminescence (PL) studies are reported for undoped and doped CdMgTe/CdSeTe double heterostructures (DHs) grown by molecular beam epitaxy. Undoped DHs are studied with absorber layer thickness varying from 0.5 to 2.5 µm. The n-type free-carrier concentration is varied ~7 × 1015, 8.4 × 1016, and 8.4 × 1017 cm−3 using iodine as a dopant in different absorber layer thicknesses (0.25–2.0 µm). Optical injection is varied from 1 × 1010 to 3 × 1011 photons/pulse/cm2, corresponding to the initial injection of photo-carriers up to ~8 × 1015 cm−3, to examine the effects of excess carrier concentration on the PL lifetimes. Undoped DHs exhibit an initial rapid decay followed by a slower dependence with carrier lifetimes up to ~485 ns. The dependence of carrier lifetimes on the thickness of the absorber layers (0.5–2.5 µm) suggests interface recombination velocities () ~ 1288 and 238 cm s−1 in the initial and later decay times, respectively, corresponding to high and low photo-carrier concentrations. The Shockley–Read–Hall model is used to describe the results in which variations are observed in for undoped DHs. The lifetimes of doped DHs show a consistent trend with thickness. The ~ 80–200 cm s−1 is estimated for doping n ~ 7 × 1015 cm−3 and 240–410 cm s−1 for n ~ 8.4 × 1016 cm−3. The observed decrease in carrier lifetimes with increasing n is consistent with growing importance of the radiative recombination rate due to the excess carrier concentration. The effect of carrier concentration on the PL spectrum is also discussed.

Effect of strain and confinement on the effective mass of holes in InSb quantum wells

The relationships between Mg composition, band gap, and lattice characteristics are investigated for Cd1−xMgxTe barrier layers using a combination of cathodoluminescence, energy dispersive x-ray spectroscopy, variable angle spectral ellipsometry, and atom probe tomography. The use of a simplified, yet accurate, variable angle spectral ellipsometry analysis is shown to be appropriate for fast determination of composition in thin Cd1−xMgxTe layers. The validity of using high-resolution x-ray diffraction for CdTe/Cd1−xMgxTe double heterostructures is discussed. The stability of CdTe/Cd1−xMgxTe heterostructures are investigated with respect to thermal processing.