Patent Document

RELATED APPLICATIONS 
     The following applications of the common assignee, which are hereby incorporated by reference in their entirety, may contain some common disclosure and may relate to the present invention: 
     U.S. patent application Ser. No. 10/284,360, entitled “EFFICIENT SPIN INJECTION INTO SEMICONDUCTORS”; and 
     U.S. patent application Ser. No. 10/284,183, entitled “MAGNETIC SENSOR BASED ON EFFICIENT SPIN INJECTION INTO SEMICONDUCTORS”. 
     FIELD OF THE INVENTION 
     This invention relates generally to spintronics and photonics. In particular, the invention relates generally to injection hetero lasers and light-emitting diodes, i.e., source of stimulated and spontaneous polarized light, based on solid-state heterostructures and efficient spin injection into semiconductors, especially at room temperature. 
     BACKGROUND OF THE INVENTION 
     Over the past decade a pursuit of solid state ultrafast scaleable devices based on both the charge and spin of an electron has led to a development of new fields of magnetoelectronics and spintronics. The discovery of giant magnetoresistance (GMR) in magnetic multilayers has quickly led to important applications in storage technology. GMR is a phenomenon where a relatively small change in magnetism results in a large change in the resistance of the material. 
     The phenomenon of a large tunnel magnetoresistance (“TMR”) of ferromagnet-insulator-ferromagnet (“F 1 -I-F 2 ”) structures is a focus of product development teams in many leading companies. TMR is typically observed in F 1 -I-F 2  structures made of two ferromagnetic layers, F 1  and F 2 , of similar or different materials separated by the insulating thin tunnel barrier I with thickness typically ranging between 1.4–2 nm. 
     It is worth mentioning recent studies of the giant ballistic magnetoresistance of Ni nanocontacts. Ballistic magnetoresistance is observed in Ni and some other nanowires where the typical cross-section is a few square nanometers. The transport in this case is through very short constriction made on the vicinity of the nanowire and it is thought to proceed with conservation of electron momentum (ballistic transport). The change in the contact resistance can exceed 10 fold (or over 1000%). 
     Of particular interest has been the injection of spin-polarized carriers, mainly in the form of spin-polarized current into semiconductors. This is significant due to relatively large spin-coherence lifetime of electrons in semiconductors, including possibilities for use in hetero laser and light-emitting diodes of polarized radiation. Development of sources of stimulated and spontaneous polarized radiation, i.e., laser and a light-emitting diode of polarized light is one of the most urgent problems of optical communication. Conventional sources have low degree of polarization. 
       FIG. 1A  illustrates a schematic prototypical model of a conventional double hetero laser and light-emitting diode  100 . As shown, the diode  100  includes a first semiconductor layer  110 , a second semiconductor layer  120  below the first semiconductor layer  110 , and a third semiconductor layer  130  below the second semiconductor layer  120 . The diode  100  also includes a substrate  140  below the third semiconductor layer  130  and first and second contacts  150  and  160  above the first semiconductor layer  110  and below the substrate  140 , respectively. 
     The first semiconductor layer  110  is relatively heavily negatively doped (n + ) and the third semiconductor layer  130  is relatively heavily positively doped (p + ). The second semiconductor layer  120  may be either positively (p) or negatively (n) doped, but as a rule, the dopant concentration level is less than that of the first or the third semiconductor layers  110  or  130 . Main feature of the double heterostructure is that the second semiconductor layer  120  has the narrower band gap when compared to the band gaps of the adjacent first and second semiconductor layers  110  and  130 . 
       FIG. 1B  illustrates an energy band diagram of the diode  100  illustrated in  FIG. 1A  along the line I—I at equilibrium. In this figure, the Fermi level E F , the bottom conduction band energy level E C , and the top valence band energy level E V  are shown. Also, the energy band gaps for each material E g1  (first semiconductor layer  110 ), E g2  (second semiconductor layer  120 ), and E g3  (third semiconductor layer  130 ) are shown where E gi =E Ci −E Vi  for each layer, i=1–3. As mentioned above, E g2 &lt;E g1 , E g3 . 
       FIG. 1C  shows the same as  FIG. 1B , but at a large bias. Radiation in the diode  100  is generated as a result of radiative recombination of non-equilibrium electrons and holes in the second semiconductor layer  120 . The electrons and holes are injected into the second semiconductor layer  120  (which has narrower band gap) from first and third semiconductor layers  110  and  130 , respectively. The electrons and holes are only slightly spin-polarized (due to weak spin-orbital coupling) in the conventional light-emitting heterostructure such as the diode  100 . Consequently, the radiation has a very low degree of polarization. 
     The possibility of spin injection from ferromagnetic semiconductors (FMS) into nonmagnetic semiconductors has been demonstrated in a number of recent publications. However, the Curie temperature (the temperature above which a material becomes nonmagnetic) of magnetic semiconductors is substantially below room temperature. The low Curie temperature limits possible applications. Room-temperature spin injection from ferromagnets (FM) into semiconductors has also been demonstrated, but its efficiency is very low (˜1–2%). 
     The main problem of the spin injection from a ferromagnet into semiconductor is that a potential barrier (Schottky barrier with the height Δ) for carriers always forms in the semiconductor near the metal-semiconductor interface due to different values of the electrode work function and the affinity of a semiconductor. Numerous experiments have shown that the barrier height Δ is determined by surface states forming at the interface, which is approximately (⅔)E g  almost independent of the type of a metal, where E g  is the energy band gap of the semiconductor. For example, in GaAs E g ≈1.42 eV and Δ≈0.8–1.0 eV, in the case of Si E g =1.12 eV and Δ≈0.6–0.8 eV. 
     SUMMARY OF THE INVENTION 
     According to an embodiment of the present invention, a hetero-laser and light-emitting device comprises a first semiconductor layer being doped with a first dopant type; a second semiconductor layer being doped with a second dopant type and being formed below the first semiconductor layer; a third semiconductor layer being doped with the second dopant type and being formed below the second semiconductor layer; a ferromagnetic layer formed above the first semiconductor layer; a first δ-doped semiconducting layer being doped with the first dopant type and being formed between the ferromagnetic layer and the first semiconductor layer; and a second δ-doped semiconducting layer being doped with the first dopant type and being formed between the first semiconductor layer and the second semiconductor layer. 
     According to another embodiment of the present invention, a method to form a hetero-laser and light-emitting device comprises forming a first semiconductor layer doped with a first dopant type; forming a second semiconductor layer doped with a second dopant type and located below the first semiconductor layer; forming a third semiconductor layer doped with the second dopant type and located below the second semiconductor layer; forming a ferromagnetic layer above the first semiconductor layer; forming a first δ-doped layer doped with the first dopant type and located between the ferromagnetic layer and the first semiconductor layer; and forming a second δ-doped layer doped with the first dopant type and located between the first semiconductor layer and the second semiconductor layer. 
     According to a further embodiment of the present invention, a method of emitting polarized light comprises providing a hetero-laser and light-emitting device, wherein the devices comprises a first semiconductor layer doped with a first dopant type, a second semiconductor layer being doped with a second dopant type and located below the first semiconductor layer, a third semiconductor layer being doped with the second dopant type and located below the second semiconductor layer, a ferromagnetic layer above the first semiconductor layer, a first δ-doped layer doped with the first dopant type and located between the ferromagnetic layer and the first semiconductor layer, and a second δ-doped layer doped with the first dopant type and located between the first semiconductor layer and the second semiconductor layer; and applying a bias voltage between the ferromagnetic layer and the substrate. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Features of the present invention will become known from the following description with reference to the drawings, in which: 
         FIG. 1A  illustrates a schematic of a conventional double hetero laser and light-emitting diode; 
         FIG. 1B  illustrates an energy band diagram of the double hetero laser and light-emitting diode shown in  FIG. 1A  along the line I—I at equilibrium; 
         FIG. 1C  illustrates an energy band diagram of the double hetero laser and light-emitting diode shown in  FIG. 1A  along the line I—I when a relatively large bias voltage is applied to the diode; 
         FIG. 2A  illustrates a density of electronic states (DOS) of ferromagnetic Ni; 
         FIG. 2B  illustrates the density of electronic states (DOS) of ferromagnetic Ni, but at a higher resolution than in  FIG. 2A ; 
         FIG. 3A  illustrates a hetero laser and light-emitting device according to an embodiment of the present invention; 
         FIG. 3B  illustrates an exemplary energy diagram of the device shown in  FIG. 3A  along the line III—III, at equilibrium; 
         FIG. 3C  illustrates an exemplary energy diagram of device shown in  FIG. 3A  along the line III—III, at bias; 
         FIG. 3D  illustrates an exemplary energy diagram of device shown in  FIG. 3A  along the line III—III, at equilibrium, wherein the δ-doped layers have narrower energy band gaps than that of the semiconductors; 
         FIG. 3E  illustrates an exemplary energy diagram of device shown in  FIG. 3A  along the line III—III, at bias, wherein the δ-doped layers have narrower energy band gaps than that of the semiconductors; 
         FIG. 3F  illustrates a hetero laser and light-emitting device according to another embodiment of the present invention; 
         FIGS. 4A–4C  illustrate an exemplary method of manufacturing the hetero laser and light-emitting device shown in  FIG. 3A  according to an embodiment of the present invention; and 
         FIGS. 5A–5E  illustrate an exemplary method of manufacturing the hetero laser and light-emitting device shown in  FIG. 3F  according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     For simplicity and illustrative purposes, the principles of the present invention are described by referring mainly to exemplary embodiments thereof. In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. However, the present invention may be practiced without limitation to these specific details. In other instances, well known methods and structure have not been described in detail so as not to unnecessarily obscure the present invention. 
     The density of states (“DOS”) is one of main characteristics of electrons in solid states, in particular, in magnetic materials, such as ferromagnetic Ni, Co, and Fe. DOS is defined as g i (E)dE, which is the number of electron states characterized by some quantum number i per unit volume in an energy interval (E, E+dE).  FIG. 2A  illustrates the DOS of ferromagnetic Ni, where arrows indicate the DOS for majority (d-electrons with spin up ↑, d↑) and minority (spin down, d↓) electrons, together with the DOS for s- and p-electrons. Note that the DOS have high peaks for both spin-up and spin-down electrons at certain energy intervals. 
       FIG. 2B  illustrates the DOS of ferromagnetic Ni, but at a higher resolution than in  FIG. 2A . The energy origin is chosen at the Fermi level E F , i.e. E=E F =0. As shown, there is a very large difference in the DOS of minority and majority d-electrons at E&gt;0 (states above the Fermi level). The peak in the DOS of minority d-electron states is positioned at E=Δ 0 , which for Ni, Δ 0 ≈0.1 eV. Similar region at E&gt;0 exists in Co and Fe. Note that near E≈Δ 0 , the DOS of the majority d-electrons and DOS of s- and p-electrons are all negligible when compared with the DOS of minority d-electrons. Thus, if electrons are injected from the ferromagnetic material with energies E≈Δ 0 , the electrons would be almost 100% polarized. 
       FIG. 3A  illustrates a hetero laser and light-emitting device  300  according to an embodiment of the present invention. As shown, the device  300  may include a first semiconductor layer  310 , a second semiconductor layer  320  below the first semiconductor layer  310 , and a third semiconductor layer  330  below the second semiconductor layer  320 . The device  300  may also include a magnetic layer  370  above the first semiconductor layer  310 , a first δ-doped semiconductor layer  315  between the magnetic layer  370  and the first semiconductor layer  310 , and a second δ-doped semiconductor layer  325  between the first semiconductor layer  310  and the second semiconductor layer  320 . The device  300  may further include a substrate  340  below the third semiconductor layer  330 , and first and second contacts  350  and  360  above the magnetic layer  370  and below the substrate  340 , respectively. 
     The first semiconductor layer  310  may be relatively heavily negatively doped (n + ), and both the second and third semiconductor layers  320  and  330  may be relatively heavily positively doped (p + ). In an embodiment, the energy band gap of the second semiconductor layer  320 , E g2 , is less than the energy band gaps of the first or third semiconductor layers  310  or  330 , E g1  or E g3  as shown in  FIG. 3B . 
     The second semiconductor layer  320  may be formed from semiconductors with direct optical transitions. In such semiconductors, an electron can directly recombine with a hole without emitting/absorbing photon. Second semiconductor layer  320  may be formed, for example, from materials such as GaAs, AlGaAs, InGaAs, InGaPAs, InAs, GaSb, InSb, InGaSb, AlAs, AlSb, ZnTe, CdTe, HgCdTe, and alloys which may include various combinations of these materials. 
     In an embodiment, the thickness w of the second semiconductor layer  320  is less than a diffusion length of non-equilibrium carriers in this layer. The majority semiconductors with direct optical transitions, such as the ones listed above, may be characterized by two types of holes: light holes with an effective mass m pl  and heavy holes with an effective mass m ph &gt;&gt;m pl . The light and heavy holes may be typically characterized by different effective spin projections, 
         μ   hl     =         ±     1   2       ⁢           ⁢   and   ⁢           ⁢     μ   hh       =     ±       3   2     .             
 
Therefore, during recombination of the holes with the spin-polarized electrons 
       (       with   ⁢           ⁢     μ   e       =     1   2       )       
 
the light is generated with polarization P=±1.
 
     In an embodiment, to increase the degree of the radiation polarization, one type of the holes, such as the light holes, are excluded from the recombining. This may be achieved by means of size quantization of the hole levels in the second semiconductor layer  320 , which is a “quantum well”. (See  FIG. 3C ). Reducing the thickness w of the second semiconductor layer  320  achieves appreciable quantization of energy of the light holes in the potential well of the p +  second semiconductor layer  320 . The lower energy level may be higher than the thermal energy k B T, where T is the temperature and k B  is the Boltzmann constant. Thus, the thickness w may satisfy the following conditions:
 
w&lt;L D2   (1)
 
and
 
 w   0   &gt;w≧w   0 √{square root over ( m   pl   /m   ph )}, where  w   0   =h /√{square root over (2 m   pl   k   B   T )}  (2)
 
     As noted above, the first semiconductor layer  310  may be relatively strongly negatively doped (n + ). Also as noted above, the first and third semiconductor layer  310  and  330  may have an energy band gaps that is wider than the energy band gap of the second semiconductor layer, i.e. E g1 &gt;E g2 , E g3 &gt;E g2 . One way to accomplish this is to form the first, second, and third semiconductor layers  310 ,  320 , and  330  from double heterostructures. Examples of double heterostructures include Al y Ga 1-y As—GaAs—Al x Ga 1-x As and In y Ga 1-y As—InGaAs—In x Ga 1-x As, where x and y refer to the chemical composition of the relevant materials. Typically, x≈0.125–0.2 and y≈0.2–0.3. 
     It is noted that various dopants may be used to dope the first, second, and third semiconductor layers  310 ,  320 , and  330 . Generally, various impurities may be used as electron donors and acceptors in different semiconductor materials. For the majority of direct-gap semiconductors such as GaAs, GaAsAl, InGaAs, Zn and Cd may be used to positively dope the second and third semiconductor layers  320  and  330 . Also, materials such as Ge, Se, Te, Si, Pb, and Sn may be used to negatively dope the first semiconductor layer  310  made of the same compound semiconductors. 
     In an embodiment, the thickness d of the first semiconductor layer  310  be much smaller than the spin diffusion length of electrons in the first semiconductor layer  310  such that d&lt;&lt;L eS =√{square root over (D e τ eS )}, where τ eS  is the relaxation time of electron spin and D e  is the electron diffusion coefficient of the first semiconductor layer  310 . 
     The ferromagnetic layer  370  may be formed from various magnetic materials such as Ni, Fe and Co, as well as various magnetic alloys, which may include one or more combinations of Fe, Co, Ni. In an embodiment, the thickness of the ferromagnetic layer  370  is substantially at 4–6 nm or greater but also less than the typical width of magnetic domain wall. 
     Both the first and the second δ-doped layers  315  and  325  may be heavily negatively doped (n + ) and very thin (the conditions are described below). One or both of the δ-doped layers  315  and  325  may be formed by delta-doping portions of the first semiconductor layer  310 . In other words, lower and upper portions of the first semiconductor layer  310  may be heavily doped with electron-rich materials. For example, if the first semiconductor layer  310  is formed from GaAs, materials such as Ge, Se, Te, Si, Pb, and Sn may be used as dopants. 
     The device  300  thus formed may be described as having a FM1-n δ1   + -n 1 -n δ2   + -p 2   + -p 3   +  structure corresponding to the layers  370 ,  315 ,  310 ,  325 ,  320 , and  330 , respectively. An example of such structure is Ni-n δ1   + -Ga 0.875 Al 0.125 As-n 1 -Ga 0.875 Al 0.125 As-n δ2   + -Ga 0.875 Al 0.125 As-p 2   + -GaAs-p 3   + -Ga 0.8 Al 0.2 As. In other words, in this example, the second semiconductor layer  320  is formed from GaAs. Also, the first and third semiconductor layers  310  and  330  and the first and second δ-doped layers  315  and  325  are all formed from GaAlAs with composition parameters x and y being 0.125 and 0.2, respectively. Other example structures include Ni—GaAs—GaAs—GaAs—In x Ga 1-x As—GaAs; Ni—GaAs—GaAs—GaAs—In x Ga 1-x As—GaAs; Ni(Fe)—CdTe—CdTe—CdTe—Cd x Hg 1-x Te—CdTe; and Ni(Fe)—Zn x Cd 1-x Se—ZnSe—Zn x Cd 1-x Se—Zn—ZnyCd 1-y Se. 
       FIGS. 3B and 3C  illustrate exemplary energy diagrams of the device  300  shown in  FIG. 3A  along the line III—III, at equilibrium and at bias, respectively. In this embodiment, the first and second δ-doped layers  315  and  325  are assumed to be formed by delta-doping the respective portions of the first semiconductor layer  310 . In  FIG. 3B , the Fermi level E F , the bottom conduction band energy level E C , and the top valence band energy level E V  are shown. The energy origin is chosen at the Fermi level, in other words, E F  is defined to be at zero. Also, the energy band gaps for each material E g1  (first semiconductor layer  310 ), E g2  (second semiconductor layer  320 ), and E g3  (third semiconductor layer  330 ) are shown where E gi =E Ci −E Vi  for each layer.  FIG. 3C  shows the same as  FIG. 3B , but under a bias voltage. It is clear from that the potential well forms in the second semiconductor layer  320  under bias voltage. 
     In an embodiment, the first δ-doped layer  315  screens the Schottky barrier at interface between the ferromagnetic layer  370  and the first semiconductor layer  310  so that it becomes transparent for tunneling electrons. In other words, the electrons may easily traverse the first δ-doped layer  315 . The second δ-doped layer  325  may screen the interfacial potential barrier between the first and second semiconductor layers  310  and  320 , so that it becomes transparent for tunneling electrons. If the following conditions are satisfied, the electrons may easily traverse the first and second δ-doped layers  315  and  325 , i.e. be transparent: 
                         N   d1     ⁢     l     +   1     2       ≈     2   ⁢         ɛ   0     ⁢     ɛ   ⁡     (       Δ   1     -     Δ   3       )           q   2           ,                             l     +   1       ≤     t   1       =         ℏ   2       2   ⁢       m   *     ⁡     (       Δ   1     -     Δ   3       )               ,                 (   2   )                           N   d2     ⁢     l     +   2     2       ≈     2   ⁢         ɛ   0     ⁢   ɛ   ⁢           ⁢     Δ   2         q   2           ,                                                       ⁢           l     +   2       ≤     t   2       =         ℏ   2       2   ⁢     m   *     ⁢     Δ   2             ,                   (   3   )             
 
where N d1  and N d2  represent donor concentrations of the first and second δ-doped layers  315  and  325 , respectively; l +1  and l +2  represent the thicknesses of the first and second δ-doped layers  315  and  325 , respectively; ε 0  represents the permittivity of free space; ε represents a relative permittivity of the first semiconductor layer  310 ; Δ 1  represents the height of the Schottky barrier (as measured from the Fermi level of the ferromagnetic layer  370 ) at the boundary between the ferromagnetic layer  370  and the first δ-doped layer  315 ; Δ 3  represents the height of the lower and wider potential barrier in the first semiconductor layer  310  (also measured from Fermi level of the ferromagnetic layer  370 ); Δ 2  represents the step of the potential barrier at the interface between the first and second semiconductor layers  310  and  320 ; q represents elementary charge; h is the Planck&#39;s constant, and m* represents an effective mass of electron of the first and second δ-doped layers  315  and  325 . Typically, the thicknesses l +1 ≈l +2 ≈(1–2) nm and the donor concentrations N d1  and N d2  may be greater than or substantially equal to (10 19 –10 20 ) cm −3 .
 
     The electrons that tunnel through the relatively high potential barrier Δ 1  of the thin first δ-doped layer  315  with the energy E&gt;E F  face another potential barrier formed in the first semiconductor layer  310 , which is shallow (barrier height Δ 3 ) and much wider (of thickness, d&gt;&gt;l +1 ). In an embodiment, the width d of the first semiconductor layer  310  be wide enough, yet d&lt;&lt;L D1 , where L D1  is the diffusion length of carriers of the first semiconductor layer  310 . When this occurs, electrons with energies below the barrier height Δ 3  are effectively filtered and, essentially, only the electrons with energies above the barrier height E&gt;Δ 3  will be able to traverse the length of first semiconductor layer  310 . 
     As will be explained below, in an embodiment, the height of the barrier Δ 3  in the first semiconductor layer  310  coincides with the peak DOS for the minority d-electrons (see  FIGS. 2A and 2B ). Note that the potential barrier Δ 3  in the first semiconductor layer  310  may be manipulated, for example by controlling the donor concentration N d1  of the first semiconductor layer  310 . As previously noted, the DOS of minority d-electrons of the ferromagnetic layer  370  reaches maximum at energy level E≈E F +Δ 0  (see  FIGS. 2A and 2B ). For simplicity, origin is chosen such that E F =0. Then, at E≈Δ 0 , the maximum DOS of minority d-electrons exceeds, by more than an order of magnitude, the DOS of electrons for all other types. 
     Thus, if the potential barrier height of the first semiconductor layer  310  is such that it coincides with Δ 0  (Δ 3 ≈Δ 0 ), then the electrons from ferromagnetic layer  370  tunneling through the first δ-doped layer  315  and traversing the length d of the first semiconductor layer  310  will be composed of almost all minority d-electrons. In other words, the injected current will be almost 100% spin-polarized. 
     With reference to  FIG. 3C , the operation of the device  300  is explained as follows. Under bias, almost 100% spin-polarized electrons are efficiently injected from the ferromagnetic layer  370  through the n + -doped first δ-doped layer  315  into the n-doped first semiconductor layer  310 . When the thickness d of the first semiconductor layer  310  is much less than diffusion length L D1  of non-equilibrium carriers in this layer, the spin polarized electrons traverse the first semiconductor layer  310  and the n + -doped second δ-doped layer  325  and accumulate in the thin narrower band gap p + -doped second semiconductor layer  320 . Simultaneously, holes are injected from the wide gap p + -doped third semiconductor layer  330  into the second semiconductor layer  320  and the heavy holes (with projections of the effective spin 
           μ   hh     =     ±     3   2         )       
 
accumulate there, blocked by the energy barrier Δ 4 , provided that Δ 4 &gt;&gt;k B T.
 
     Highly polarized light is emitted due to radiative recombination of the holes with accumulated spin polarized electrons. This occurs when the spontaneous or stimulated radiation lifetime is less than the spin relaxation time of the electrons in the second semiconductor layer  320 . This may be realized when concentration of injected electrons n in the layer  320  is relatively high, for example, above 10 17  cm −3 . 
     Note that the minimal energy of the light holes (those with projections of the effective spin 
           μ   hl     =     ±     1   2         )       
 
in the quantum well  320  exceeds k B T by design, so they cannot accumulate in the layer  320 . The electrons with 100% spin polarization (with projection 
           μ   e     =     1   2       )       
 
can only recombine with heavy holes, according to selection rule for angular momentum, in the channel μ e +μ hh =−1, since the photon polarization can only take the value P=−1. Another channel, μ e +μ hh =2, is prohibited as well. Therefore, the emitted photons will all have the polarization P=−1, i.e. the radiation will be almost 100% polarized.
 
     In another embodiment of the present invention, one or both first and second δ-doped layers  315  and  325  may be formed by growing a n + -doped epitaxial layer on the n-doped first and second semiconductor layers  310  and  320 . The epitaxially grown δ-doped layers  315  and/or  325  are doped heavily as practicable and be as thin as practicable. In an embodiment, one or both of the first and second δ-doped layers  315  and  325  have a narrower energy band gap than the energy band gap of the first semiconductor layer  310  and that electron affinities of the δ-doped layers  315  and  325  be greater than an electron affinity of the first semiconductor layer  310  by a value close to Δ 0 . 
     If the δ-doped layer  315  is formed by epitaxial growth of a very thin heavily doped (i.e. n +  doped) and narrower energy band gap semiconductor layer, the parameters of the first δ-doped layer  315  i.e. its donor concentrations N d  and its thickness l +1  should satisfy the following conditions: 
                       N   d1     &gt;     2   ⁢         ɛ   0     ⁢     ɛ   ⁡     (       Δ   1     -     Δ   3       )             q   2     ⁢     l     +   1     2             ,                         l     +   1       ≤     t   1                   (   4   )             
 
     The device  300  thus formed may also be described as having a FM1-n δ1   + -n 1 -n δ2   + -p 2   + -p 3   +  structure corresponding to the layers  370 ,  315 ,  310 ,  325 ,  320 , and  310 , respectively. An example of such structure is FM1-Ni-n δ1   + -GaAs-n 1 -Ga 1-x Al x As-n δ2   + -GaAs-p 2   + -GaAs-p 3   + -Ga 1-x Al x As. In other words, in this example, the n + -doped first and second δ-doped layers  315  and  325  and the second semiconductor layer  320  are formed from GaAs and the first and third semiconductor layers  310  and  330  are formed from Ga 1-x Al x As. Other example structures include Ni—In 1-x Ga x As—GaAs—In 1-x Ga x As—GaAs—GaAs; Ni(Fe)—Cd x Hg 1-x Te—CdTe—Cd x Hg 1-x Te—CdTe—Cd x Hg 1-x Te; and Ni(Fe)—Zn x Cd 1-x Se—ZnSe—Zn x Cd 1-x Se—ZnSe—Zn x Cd 1-x Se. As noted previously, the first and second δ-doped layers  315  and  325  should be transparent to tunneling electrons. This condition may be satisfied, for example, if the first and second δ-doped layers  315  and  325  are such that the thickness l +1,2 ≦(1–2) nm and the donor concentration N d1   + ≧10 20  cm −3  and N d2 ≧10 19  cm  −3.    
       FIGS. 3D and 3E  illustrate exemplary energy diagrams of the device  300  shown in  FIG. 3A  along the line III—III, at equilibrium and under bias voltage, respectively. In this embodiment, the first δ-doped layer  315  is assumed to be formed by epitaxial growth of narrower energy band gap semiconductor. The operation of this device  300  is similar to that as shown in  FIGS. 3B and 3C , but the efficiency of the device may be even greater. 
       FIG. 3F  illustrates another a hetero laser and light-emitting structure  300 - 2  according to another embodiment of the present invention. The device  300 - 2  is similar to the device  300  shown in  FIG. 3A , except that the first and second electrical contacts  350  and  360  are placed as shown. The operation of the device  300 - 2  is similar and need not be repeated here. The electrical contact  350  and  360  are placed as shown. The bottom electrode  360  can be made magnetic, FM2, to inject spin-polarized holes through the second semiconductor layer  320  (p + -S 2 ). In an embodiment, the thickness of this layer is much smaller than the spin diffusion length of holes in the semiconductor layer  320 , W&lt;&lt;L hs =√{square root over (D h τ hS )}, where τ hS  is the relaxation time of hole spin and D h  is the hole diffusion coefficient in the third semiconductor layer  330 . 
       FIGS. 4A–4C  illustrate an exemplary method of manufacturing the device  300  shown in  FIG. 3A . As shown in  FIG. 4A , the contact second contact  360  and the substrate  340  may be formed. The substrate  340  may be planarized. Then the third semiconductor layer  330  may be formed on the substrate  340  and the second semiconductor layer  320  may be formed on the third semiconductor layer  330  may be formed by epitaxial or molecular growth. Materials to form the third semiconductor layer  330  may be deposited, sputtered, fired on the substrate  340 . Likewise, the second semiconductor layer  320  may also be deposited, sputtered, fired on the third semiconductor layer  330 . One or both of the third and second semiconductor layers  330  and  320  may be planarized. 
     Then as shown in  FIG. 4B , the first and second δ-doped layers  315  and  325  and the first semiconductor layer  310  may be formed. In one embodiment, the second δ-doped layer  325  may be formed by epitaxial or molecular growth. The second δ-doped layer  325  may also be deposited, sputtered, or fired onto the second semiconductor layer  320 . Then the first semiconductor layer  310  may be deposited, fired, or sputtered onto the second δ-doped layer  325 . Then the first δ-doped layer  315  may be formed by epitaxial or molecular growth, or may be deposited (e.g., by molecular deposition, liquid epitaxy, or MOCVD), sputtered, or fired onto the first semiconductor layer  310 . Note that each of the first and second δ-doped layers  315  and  325  and the first semiconductor layer  310  may be planarized. Also, the first and second δ-doped layers  315  and  325  may be doped more heavily as compared to the first semiconductor layer  310 . 
     In another embodiment, the first semiconductor layer  310  may be formed on the second semiconductor layer  320  and the first and second δ-doped layers  315  and  325  may be formed by heavily doping appropriate portions of the first semiconductor layer  310  or by epitaxial or molecular growth. 
     Then as shown in  FIG. 4C , the ferromagnetic layer  370  may be formed, again by epitaxial or molecular growth, or may be deposited, sputtered, or fired onto on the first δ-doped layer  315 . The ferromagnetic layer  370  may be planarized. Then as shown, the first electrode  360  may be formed by sputtering, firing, or depositing materials on the ferromagnetic layer  370 . 
       FIGS. 5A–5D  illustrate an exemplary method of manufacturing the device  300 - 2  shown in  FIG. 3F . As shown in  FIG. 5A , the substrate  340  may be formed and the contact second contact  360  may be formed on the substrate  340 . The second contact  360  may be deposited, sputtered, fired on the substrate  340  and may be planarized. The second contact  360  may be from a ferromagnetic material. Then the third semiconductor layer  330  may be formed on the second contact  360  and the second semiconductor layer  320  may be formed on the third semiconductor layer  330 . Materials to form the third semiconductor layer  330  may be deposited, sputtered, fired on the substrate  340 . Likewise, the second semiconductor layer  320  may also be deposited, sputtered, fired on the third semiconductor layer  330 . One or both of the third and second semiconductor layers  330  and  320  may be planarized. 
     Then as shown in  FIGS. 5B and 5C , the steps the form the first and second δ-doped layers  315  and  325 , the first semiconductor layer  310 , the ferromagnetic layer  370 , and the electrical contact  350  are similar to the steps shown in  FIGS. 4B and 4C , and thus the details need not be repeated here. 
     Then the contact  350 , ferromagnetic layer  370 , first and second δ-doped layers  315  and  325 , and first and second semiconductor layers  310  and  320  are etched to expose the third semiconductor  330  as shown in  FIG. 5D . The etched areas are then filled with oxides  380  as shown in  FIG. 5E . 
     What has been described and illustrated herein are preferred embodiments of the invention along with some of its variations. The terms, descriptions and figures used herein are set forth by way of illustration only and are not meant as limitations. Many variations are possible within the spirit and scope of the invention, which is intended to be defined by the following claims—and their equivalents—in which all terms are meant in their broadest reasonable sense unless otherwise indicated.

Technology Category: 5