Semiconductor devices and methods

Details Semiconductor devices and methods Semiconductor devices and methods

2002-07-31

2004-06-22

Nhu, David (Department: 2818)

Semiconductor device manufacturing: process

Quantum dots and lines

C438S045000, C438S604000

Reexamination Certificate

active

06753273

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to the field of semiconductor devices and methods of making same, and, more particularly, to a semiconductor device having a quantum well and a layer of quantum dots spaced from the quantum well, and to a method of making such a device.
BACKGROUND OF THE INVENTION
In 1977 there was introduced the quantum-well (QW) laser, a laser that in the active region employs one or more thin quantum well layers (L
z
less than about 400 Å) sandwiched in a thicker waveguide region that is further enclosed by still higher gap p-type and n-type confining layers (see E. A. Rezek, N. Holonyak, Jr., B. A. Vojak, G. E. Stillman, J. A. Rossi, D. L. Keune, and J. D. Fairing, “LPE In
1-x
Ga
x
P
1-z
As
z
(x~0.12,z~0.26) DH Laser With Multiple Thin-Layer (<500 Å) Active Region,” Appl. Phys. Lett., vol 31, pp 288-290, Aug. 15, 1977; E. A. Rezek, H. Shichijo, B. A. Vojak, and N. Holonyak, Jr., “Confined-Carrier Luminescence of a Thin In
1-x
Ga
x
P
1-z
As
z
Well (x~0.13, z~0.29; ~400 Å) in an InP p-n Junction,” Appl. Phys. Lett., vol. 31 pp. 534-536, Oct. 15, 1977). For about ten years, numerous groups have been trying to convert the quantum well laser, which is practical and widely manufactured, into a so-called quantum-dot laser. The goal has been to build an ultimate laser. In theory, the thin quantum well layer (or layers) of a QW laser is “cut-up” into (ideally) a uniform dense sheet of identical little quantum boxes (i.e., quantum dots). With such a structure, instead of the electrons and holes being injected (via current, light, etc.) and collected in an easily grown thin (L
z
less than about 400 Å) uniform QW sheet (thus with one dimensional thickness confinement and quantization; a 2-D structure), the electrons and holes are collected and recombine in the little quantum dots (i.e., quantum boxes; hence, three-dimensional confinement and quantization; a O-D structure). The little boxes, or dots, are like “fat” little molecules and (ideally) all alike and densely and uniformly arranged. [Reference can be made, for example, to U.S. Pat. No. 6,369,403, and to J. H. Ryou, R. Dupuis, G. Walter, D. Kellog, N. Holonyak, Jr., D. Mathes, R. Hull, C. Reddy, and v. Narayanamuri, Appl. Phys. Lett. 78, 4091 (2001).] Actually, the quantum dots are stochastic and probabilistic, small but random. They are random in geometry, in size, and in arrangement - - - and not dense enough, i.e., are not in sufficient proximity to be quantum coupled (by tunneling). Electron-hole (e-h) pairs are stuck in each box (assuming they are collected at all) and don't transport to neighboring boxes. The e-h pairs don't move around and rearrange in the plane of the dots as readily as they do in a thin QW sheet (L
z
less than about 400 Å; L
x
, L
y
extended in the plane; hence a 2-D structure). In short, there is little or no conduction along the sheet (or sheets) of quantum dots.
It is among the objects of the present invention to provide devices, and methods of making same, which are responsive to the foregoing limitations of the prior art, and to also provide devices which exhibit improved operation as light emitters and as other applications.
SUMMARY OF THE INVENTION
A major problem with quantum dot heterostructures (QDHS) is the stochastic nature of QD structures, the lack of uniformity, insufficient QD density, and poor QD-to-QD coupling. In accordance with an aspect hereof, a solution to the problem is to couple, via a thin barrier, the sheet (or multiple sheets) of QDs to a QW layer, e.g., to the QW layer of a QW laser. The higher energy states of the QDs are resonant with and couple by tunneling to the lower confined-particle states of the thin uniform QW layer. Electron-hole transport in the QW layer can re-arrange the e-h pairs in the plane and provide the dot-to-dot coupling. That is, an electron tunnels from the QD to the QW layer, transports in the plane in the QW layer, and re-tunnels back into another QD. Hence, one can couple the QDs - - - not in the plane, but out of the plane into a QW layer, then at some other point in the plane of the QW layer back into another QD. In this manner, the invention makes it possible for the charge to get unstuck from a QD and move from dot to dot in the waveguide region to help optimize emission.
Also, should the QDs not collect injected electron-hole pairs efficiently (a distinct possibility), the thin auxiliary QW layer (or, if necessary or desirable, multiple QW layers) will collect the injected carriers and feed them via resonant tunneling into the quantum dots to then scatter the carriers down to the lower energy dot states for recombination (for photon generation and laser operation). It is clear that multiple QD layers and multiple QW auxiliary or connection (communication) layers can be employed and can be part of laser or other device designs.
In accordance with a further aspect hereof, the quantum dots, since they are a lower gap component of the III-V QD heterostructure, can be doped or be left undoped. In other words, the QD can be modified into a quantum “doping ball.”There may be advantages in employing n-type or p-type QD “doping balls”, just as there may be device-design reasons to leave the QW auxiliary connection or communication layer undoped, or doped n-type or doped p-type. The QW auxiliary connection layer can be close coupled to the QDs (small separating barrier thickness) or can be weakly coupled (large barrier thickness). Also, the QW auxiliary layer can be thicker and drop its lower confined-particle states below the lowest QD states, or can be made thinner and raise its lowest confined-particle states above the lowest QD states. The foregoing types of techniques and structures can be incorporated into field effect transistor devices. For example, doped QDs, i.e., “doping balls,” can be used to modify the so-called pseudomorphic transistor into a new form of field effect transistor. The doped QDs can be employed with or without auxiliary QW layer(s).
After GaAs (E
g
=1.42 eV) it becomes almost impossible to make a higher gap, say GaP (E
g
=2.26 eV), tunnel diode. Higher gaps (higher energies) require narrower tunneling distances, which in a p-n tunnel diode requires higher doping, not, as is common, the lower impurity solubility that comes with higher gap.
A tunnel diode can be devised that is not one large uniform junction, but rather an array of microscopic tunnel junctions, for instance an array of doped quantum dots on the n side of a pn junction. For example, the GaAs(p
+
)—InGaAs(n
+
)—GaAs(n
+
)p-n tunnel diode made in 1993 (Richard, et al., Appl. Phys. Lett. 63, 3616 (Dec 27, 1993), i.e., the narrow gap InGaAs center region, a reduced barrier, can be “cut up” into quantum dots. The significance of this kind of structure for tunnel contacts on VCSEL lasers, etc., can be understood in conjunction with U.S. Pat. No. 6,369,403.
Similar doped QDs, of a type that are employed in a tunnel contact or a tunnel diode, are the same type of
doped
QDs, or doping balls (DBs) that can be used as doping sources in any layered heterostructure, including in all the various kinds of quantum dot (QD) light emitting devices (e.g. lasers) that are described in numerous articles in the journal literature. Doping dots (DDs), or doping balls (DBs), can be laced through high gap material (in layers) to act as the source of carriers. The little QD doping ball, being small enough, can be incorporated into the higher gap material even if lattice mismatched and not create defects (dislocations). The doping ball, and its “squeezed” higher energy states, carries impurity at the solubility limits of the QD material, for example, an InP QD in higher gap In
0.5
Ga
0.5
P or in In
0.5
Al
0.3
Ga
0.2
P, and not at the solubility limits of the higher gap layer (the matrix) into which the QDs are incorporated.
The layers or regions of doping QDs (DDs or DBs) are versatile layers (n-type, p-type, or undoped) that can be incorporated into heterostructure devic

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