Short-Period GaAs/AlAs Superlattices

• Short-period GaAs/AlAs superlattices are semiconductor heterostructures with alternating GaAs and AlAs layers (1–10 nm) that form engineered electronic minibands and modified phonon spectra.
• They are fabricated using precise techniques like MBE and MOCVD to achieve atomic-layer interfaces, enabling tailored band offsets and enhanced remote-donor screening in quantum wells.
• These superlattices exhibit versatile optical, excitonic, and nonlinear electronic properties, making them ideal for ultra-high-Q microcavities, phonon quantum optics, and high-frequency oscillators.

Short-period GaAs/AlAs superlattices (SPSLs) are semiconductor heterostructures composed of alternating thin layers of gallium arsenide (GaAs) and aluminum arsenide (AlAs), with individual layer thicknesses typically in the 1–10 nm regime. These periodic stacks alter the electronic, optical, transport, and vibrational properties relative to bulk or alloyed semiconductors, supporting engineered minibands, modified phonon spectra, and tailored scattering environments. SPSLs are utilized in high-mobility modulation-doped quantum wells, ultra-high-Q Bragg mirrors in microcavities, investigations of phonon lifetimes and quantum coherent coupling, and as model systems for nonlinear dynamical studies under resonant transport conditions.

1. Structural Design and Electronic Band Engineering

SPSLs are fabricated by molecular beam epitaxy (MBE) or metal–organic chemical vapor deposition (MOCVD), achieving abrupt interfaces and atomic-layer precision. A typical period consists of one GaAs layer (e.g., 5–8 nm thick) and one AlAs layer of similar thickness, with the total period $d = d_{\mathrm{GaAs}} + d_{\mathrm{AlAs}}$ ranging from 10–16 nm (He et al., 2019, Maznev et al., 2012).

The periodicity leads to the formation of electronic minibands, described by solutions of the Kronig–Penney model:

$$\cos(k_z a_{\mathrm{SL}}) = \cos(k a) \cosh(\kappa b) - \frac{1}{2}\left[\frac{m_b k}{m_w \kappa} + \frac{m_w \kappa}{m_b k}\right] \sin(k a)\sinh(\kappa b)$$

with $a$ and $b$ the well and barrier widths, $m_w$ and $m_b$ the respective effective masses, and $k$ and $\kappa$ wavenumbers in GaAs and AlAs (Bykov et al., 2023). For SPSL barriers adjacent to quantum wells, this engineering permits precise tuning of conduction- and valence-band offsets, effective masses, density of states, and in-plane and growth-direction carrier dynamics.

When employed as "digital alloys" in distributed Bragg reflectors (DBRs), the average Al content and refractive index of the superlattice can be directly calculated as

$$x_{\mathrm{Al}}^{\mathrm{eff}} = \frac{d_{\mathrm{AlAs}}}{d_{\mathrm{GaAs}}+d_{\mathrm{AlAs}}}$$

$$n_{\mathrm{SL}}^{\mathrm{eff}} = \frac{n_{\mathrm{GaAs}} d_{\mathrm{GaAs}} + n_{\mathrm{AlAs}} d_{\mathrm{AlAs}}}{d_{\mathrm{SL}}}$$

(Stolyarov et al., 2 Dec 2025).

2. Quantum Transport and Remote Donor Scattering Suppression

A major application of SPSLs is in modulation-doped quantum wells (QWs), where they serve as barriers on either side of a GaAs well (e.g., $d_{\mathrm{QW}} = 13$ nm) (Bykov et al., 2021, Bykov et al., 2023). Two Si-$\delta$-doping layers embedded in GaAs sublayers of the SPSL supply carriers to the QW, forming a two-dimensional electron gas (2DEG). A unique feature of AlAs/GaAs SPSLs is the presence of X-valley electrons localized in AlAs layers: these bind to remote donors, forming compact dipoles and screening random Coulomb potentials from remote ionized donors.

The quantum lifetime $\tau_q$ and transport scattering time $\tau_t$ of the 2DEG are governed by the interplay of remote-donor and background-impurity scattering:

$$\frac{1}{\tau_t} = \frac{1}{\tau_{tR}} + \frac{1}{\tau_{tB}}$$

$$\tau_{tR} = \frac{8 m^*}{\hbar} \frac{(k_F d_R)^3}{n^*_R}$$

$$\tau_{qR} = \frac{2 m^*}{\hbar} \frac{k_F d_R}{n^*_R}$$

where $k_F = \sqrt{2\pi n_e}$, $d_R$ is the relevant donor-to-QW distance, $n^*_R$ the effective two-dimensional remote-donor concentration, and $n_B$ the background impurity concentration (Bykov et al., 2021, Bykov et al., 2023).

As the 2DEG density $n_e$ increases past a critical value $n_{e,c}$, the fraction of donors bound to X-valley electrons grows, leading to a rapid decrease in $n^*_R$:

$$n^*_R(n_e) = n_{RI} [1 - f(n_e)],\quad \text{with}\quad f_{ab}(n_e) = \frac{1}{1+\exp[(n_e-a)/b]}$$

This screening mechanism causes an abrupt increase in both $\tau_t$ and $\tau_q$, enhancing transport lifetimes and resulting mobilities well beyond the limits achievable in single-barrier or alloyed systems (Bykov et al., 2021, Bykov et al., 2023). At high $n_e$, $\tau_t$ ultimately saturates at the background-scattering limit, while $\tau_q$ continues to rise with further reduction of remote donor scattering.

Illumination at low temperature can trigger further occupation of X-valley states, reducing $n^*_R$, and enhancing both the quantum lifetime and mobility—a direct observation of carrier dynamics and screening interplay (Bykov et al., 2023).

3. Optical and Excitonic Properties

The absorption and luminescence spectra of short-period GaAs/AlAs superlattices are dictated by excitonic effects in an anisotropic effective-mass environment. For layer thicknesses $< 10$ nm, the strong tunneling regime applies, causing the SPSL to mimic a bulk crystal with uniaxial mass anisotropy and modified dielectric constants (Pereira, 2018).

Excitonic Coulomb binding is calculated using anisotropic effective masses:

$$E_0 = \frac{\mu_{\rm eff} e^4}{2 (4\pi\varepsilon_0)^2 \hbar^2}$$

with

$$\mu_{\rm eff} = (\mu_{\parallel}^2 \mu_{\perp})^{1/3}$$

Bound and continuum optical transitions are described by Elliott-like formulas. The oscillator strengths of bound exciton peaks and the continuum edge obey rigorous ratios:

$\frac{f_n}{f_{\rm cont}} = \frac{4}{n^3}$

(Pereira, 2018).

Carrier-density-dependent screening induces bleaching and energy shifts of higher-order exciton lines. These features offer a direct diagnostic of injected or equilibrium carrier densities through line-shape analysis. For GaAs/AlAs SPSLs at 10 K with $a_0 \simeq 11.3$ nm and $E_0 \simeq 5.2$ meV, exciton resonances can be engineered into the THz regime.

The optical design flexibility of SPSLs enables their use as digital-alloy high-index layers in DBRs, where the quarter-wave stack condition is precisely met and the refractive index is tailorable by the period’s compositional ratio (Stolyarov et al., 2 Dec 2025).

4. Phonon Spectrum: Lifetimes and Quantum Coherent Coupling

SPSLs strongly modify the vibrational (phonon) spectrum by Brillouin-zone folding and quantum confinement. Coherent zone-center longitudinal acoustic phonons (e.g., at 340 GHz in an 8 nm/8 nm superlattice) have been studied with femtosecond pump–probe techniques (Maznev et al., 2012). The observed lifetimes are set by both intrinsic (phonon–phonon scattering) and extrinsic (interface roughness, period fluctuations) mechanisms. At room temperature, the intrinsic lifetime for 340 GHz phonons is $0.95 \pm 0.07$ ns, with extrinsic processes dominating at low temperature.

Phonon decay rates for acoustic modes in SPSLs lie in the crossover between the Akhiezer ($\tau \propto \omega^{-2}$, low-frequency) and Landau–Rumer (three-phonon, high-frequency) regimes for sub-THz frequencies. SPSL phonon lifetimes are thus similar to those predicted for bulk GaAs at the same frequency but are limited by additional extrinsic scattering channels (Maznev et al., 2012).

Quantum coherent coupling (QCC) among folded phonon modes has been demonstrated: a zone-center phonon ("driving mode") couples coherently via parametric down-conversion to a pair of acoustic ("target") phonons at half frequency, described by an effective Hamiltonian

$$H_\text{int} = \hbar\, g\, (a_0 a_1^\dag a_2^\dag + a_0^\dag a_1 a_2)$$

where $g$ is the coupling strength (He et al., 2019). The resulting time-domain phonon amplitudes exhibit multi-cycle collapse and revival—coherent energy exchange between modes—when the energy-exchange rate exceeds damping rates ($g \sim 3$ dimensionless). Stronger coupling and richer QCC physics are achievable by maximizing interface sharpness and reducing period, directly leveraging SPSL design flexibility (He et al., 2019).

5. Nonlinear Electronic Dynamics and Intrinsic Chaos

Short SPSLs (N ≈ 5–25 periods) with controlled doping exhibit distinct sequential resonant tunneling phenomena. Under appropriate dc bias and doping, the collective electron dynamics traverse regimes of self-sustained GHz oscillations, period-doubling bifurcations, and intrinsic chaos (Essen et al., 2017). Temporal evolution is governed by discretized coupled ordinary differential equations for local sublattice fields $F_i$ and sheet charges $n_i$, incorporating electronic continuity and Poisson's law.

Phase diagrams in the design space of period number $N$, doping, and contact conductivity $\sigma$ show that windows for high-frequency oscillations and robust chaos are maximal at intermediate $N$ (typically 10–14), with chaos persisting across codimension-zero parameter regions. The presence and robustness of chaotic regimes depend critically on disorder: SPSLs with thinner barriers (grown for room-temperature operation) tolerate moderate disorder and may even exhibit broader chaotic windows (Essen et al., 2017). These results guide high-frequency oscillator and true random bit generator design using short-period SPSLs.

6. SPSLs as Digital Alloys in Ultra-High-Q Microcavities

SPSLs have been exploited as "digital alloys" in the DBRs of planar GaAs-based microcavities, replacing ternary Al$_x$Ga$_{1-x}$As high-index layers to achieve smooth interfaces (AFM RMS 0.1 nm), precise quarter-wave thickness, and reproducible Al fraction for index control (Stolyarov et al., 2 Dec 2025). The effective-medium picture is refined via transfer-matrix modeling, accounting for confinement and excitonic features.

In optimized structures (MC2: $d_\mathrm{GaAs}=4.0$ nm, $d_\mathrm{AlAs}=1.41$ nm), photoluminescence shows an SPSL exciton at 1.68 eV, above the polariton emission energy, minimizing absorptive loss. The achieved cavity $Q_\mathrm{exp}=5.4 \times 10^4$ substantially exceeds that of equivalent-alloy DBRs, attributed to both interface smoothing and tuning of absorption through SPSL design. Accurate $Q$ prediction mandates inclusion of quantum and excitonic corrections to the refractive index in modeling (Stolyarov et al., 2 Dec 2025).

Application	SPSL Role	Key Performance Metrics
Modulation-doped QWs	Barrier, remote-donor screening	$\mu > 200$ m$^2$/V s,
		$\tau_q$ up to several ps
Ultra-High-Q Microcavity	High-index DBR layer	$Q_\mathrm{exp}=5.4 \times 10^4$
Phonon quantum optics	Engineered phonon confinement/coupling	$g \approx 3$, $T_\mathrm{life} > 0.5$ ns
Nonlinear transport/chaos	Sequential tunneling stack	GHz oscillations, chaos

7. Future Directions and Outstanding Issues

Further progress in SPSL engineering is likely along several axes:

• Exploiting X-valley and miniband states for enhanced remote-donor screening and custom transport regimes (Bykov et al., 2021, Bykov et al., 2023).
• Integrating quantum-coherent phonons with ultrafast optoelectronics, potentially enabling phononic quantum information processing and ultrafast acoustic control (He et al., 2019).
• Extending digital-alloy SPSL strategies to other materials and cavity geometries for even higher $Q$ and further reduction of threshold in polariton lasing (Stolyarov et al., 2 Dec 2025).
• Direct mapping and exploitation of nonlinear and chaotic dynamical regimes for tunable high-frequency oscillators and random number generators (Essen et al., 2017).

Accurate characterization of interface roughness, extrinsic scattering sources, and quantum-confinement corrections remains a prerequisite for predictive device modeling and continued performance enhancement.