JP4259851B2 - Spin filter - Google Patents

Description

【００１４】
ここで、ｋ‖は波数の面内方向の成分（ヘテロ界面に平行な成分）、ｈはプランク定数、Ｅ_pは、ｋ・ｐインターラクションパラメータ、ｍ₀は自由電子の質量、Ｅ_Fは電子のフェルミエネルギー、Ｅ_{Γ 8}，Ｅ_{Γ 7}は、それぞれ、価電子帯、スピンスプリットオフ帯のバンド端のエネルギーである。
【００１５】
図２に示す矢印の長さは、フェルミエネルギーＥ_Fを基準としたΓ₈とΓ₇のエネルギー値、すなわち、Ｅ_F−Ｅ_{Γ 8}とＥ_F−Ｅ_{Γ 7}との値に対応する。また、（３）式に示されるように、ラシュバのスピン−軌道相互作用係数αは、これらの値の逆数の微分の差として計算されることがわかる。
【００１６】
ドーピングによって実現した３重障壁スピンフィルターにおいては、図２に示すように、ラシュバのスピン分離エネルギーΔ_Rの値は、±１．５ｍｅＶから±２．５ｍｅＶ程度の比較的大きな値が得られている。尚、Ｉ−Ｖ特性に基づいて得られたスピンアップ電流のピーク値を与えるエミッタ−コレクタ電圧とスピンダウン電流のピーク値を与えるエミッタ−コレクタ電圧との差としては、第１及び第３障壁層のｎ型の不純物ドーピング量Ｎ_Dが、２×１０¹⁸ｃｍ^-3であり、ｐ型の不純物ドーピング量Ｎ_Aが、６.８５×１０¹⁸ｃｍ^-3の場合において約７ｍＶの値が得られ、Ｎ_Dが４×１０¹⁸ｃｍ^-3であり、ｐ型の不純物ドーピング量Ｎ_Aが１.３７×１０¹⁹ｃｍ^-3の場合において１２ｍＶ程度の値が得られる（T. Koga, J. Nitta, and H. Takayanagi, "Spin-Filter Device Based on the Rashba Effect Using a Nonmagnetic Resonant Tunneling Diode", Physical Review Letters, Vol. 88, No. 12, 25 March 2002 pp.126601-1-126601-4.）。
【００１７】
このように、ドーピングにより実現した三重障壁スピンフィルターにおいては、非常に優れたスピン分離特性を示すことが理論的に示されたが、第２の障壁層に高濃度のｐ型不純物を制御性良くドーピングすることは、技術的には容易なことではない。また、高濃度不純物ドーピングのみによるバンドエンジニアリング自体制御性が悪く、かつ、不純物散乱などの影響も受けやすい。
【００１８】
そこで、発明者らは、半導体スピンフィルター装置を組成変調により実現することを考えた。図３に組成変調半導体スピンフィルター装置の構造例を示す模式的な断面図を、図４に、図３の構造に対応するエネルギーバンド図を示す。図３に示す半導体スピンフィルターＢは、例えば、ＩｎＰ基板３１と、その上に順に形成されｎ型にドーピングされたｎ型Ｉｎ_0.53Ｇａ_0.47Ａｓコレクタ層３３と、ｉ型Ｉｎ_0.52Ａｌ_0.48Ａｓ第１障壁層３５（厚さ６ｎｍ）と、ｉ型（Ｉｎ_0.52Ａｌ_0.48）_X（Ｉｎ_0.53Ｇａ_0.47）_1-XＡｓ第１井戸層３７（厚さ１０ｎｍ、ｘの値は、０から０.５までリニアに変調させる）と、ｉ型Ｉｎ_0.52Ａｌ_0.48Ａｓ第２障壁層４１（厚さ３.５ｎｍ）と、ｉ型（Ｉｎ_0.52Ａｌ_0.48）_X（Ｉｎ_0.53Ｇａ0.47）_1-XＡｓ第２井戸層４５（厚さ１０ｎｍ、ｘの値は、０.５から０までリニアに変調させる。）と、ｉ型Ｉｎ_0.52Ａｌ_0.48Ａｓ第３障壁層４７（厚さ６ｎｍ）と、ｎ型Ｉｎ_0.53Ｇａ_0.47Ａｓエミッタ層５１とを有しており、ｎ型Ｉｎ_0.53Ｇａ_0.47Ａｓエミッタ層５１からｉ型Ｉｎ_0.52Ａｌ_0.48Ａｓ第１障壁層３５までは、島状の領域として形成されている。ｎ型Ｉｎ_0.53Ｇａ_0.47Ａｓコレクタ層３３に対してコレクタ電極５３が、ｎ型Ｉｎ_0.53Ｇａ_0.47Ａｓエミッタ層５１に対してエミッタ電極５５が形成されている。
【００１９】
尚、コレクタ層３３からエミッタ層５１までの構造は上下が反転した構造であっても良い。図１の構造と異なり、ｉ型Ｉｎ_0.52Ａｌ_0.48Ａｓ第１障壁層３５（厚さ６ｎｍ）からｉ型Ｉｎ_0.52Ａｌ_0.48Ａｓ第３障壁層４７（厚さ６ｎｍ）までには、不純物がドーピングされていない。その代わりに、第１井戸層３７と第２井戸層４５とでは、ＩｎＰとの格子整合を保ちながら、（Ｉｎ_0.52Ａｌ_0.48）_x（Ｉｎ_0.53Ｇａ_0.47）_1-xＡｓのｘを変化させることにより組成変調層を形成している。
【００２０】
その結果、図４に示すように、第１井戸層３７と第２井戸層４５とは、伝導帯端のエネルギーＥｃが第２障壁層４１に向けて高くなる。また、図２に示すエネルギーバンド図と異なり組成を傾斜させたため、第１井戸層３７と第２井戸層４５とにおいて、厚さ方向にバンドギャップが変化する。より詳細に説明すると、第２障壁層４１に向けてバンドギャップが次第に大きくなり、フェルミエネルギーＥ_Fを基準とした伝導帯端のエネルギー値が高くなるとともに、価電子帯端のエネルギー値は低くなる。組成変調の場合には、第２障壁層４１にｐ型の不純物をドーピングしなくても、伝導帯端のエネルギー値が山型のポテンシャル構造になり、３重障壁スピンフィルターをドーピングにより実現した場合と同様の効果が期待できる。
【００２１】
図５に、組成傾斜層を利用して実現した３重障壁スピンフィルターのスピンに依存した電流−電圧特性を示す。図５に示すように、第一スピン状態にある電子による電流のピークに対応する電圧と第二スピン状態にある電子による電流のピークに対応する電圧との差は、１.５ｍＶ程度と前述の第２障壁層にｐ型の不純物をドーピングした構造（約１２ｍＶ）と比較してかなり小さくなっていることがわかる。これは、ラシュバのスピン−軌道相互作用係数αの値が、（３）式に示されるように、Ｅ_F−Ｅ_{Γ 8}とＥ_F−Ｅ_{Γ 7}との値の逆数の微分として表されるからである。
【００２２】
以上の検討により、発明者は、スピンフィルターとしての性能を向上させるためには、伝導帯のみでなく価電子帯のエネルギーバンド構造の設計が重要であると考えた。半導体ヘテロ構造に単なる組成傾斜層を形成する構造のみではなく、スピンフィルターとしての性能をさらに向上させる構造を思いついた。すなわち、半導体ヘテロ構造にイオン化されたｎ型の不純物を導入すると、導入された領域のバンド構造が歪み、ポテンシャルエネルギーが低くなる。これを利用して、第２障壁層にｎ型の不純物を導入すればよい。
【００２３】
上記考察に基づき、本発明の一実施の形態による半導体スピンフィルター装置について、図面を参照して説明する。図６は、本実施の形態による半導体スピンフィルター装置の構造を示す断面図である。図７は、図６に示す半導体スピンフィルター装置の構造のエネルギーバンド図である。図６に示すように、本実施の形態による半導体スピンフィルター装置Ｃは、例えば、ＩｎＰ基板６１と、その上に順に形成されｎ型にドーピングされたｎ型Ｉｎ_0.53Ｇａ_0.47Ａｓエミッタ層６３と、ｉ型Ｉｎ_0.52Ａｌ_0.48Ａｓ第１障壁層６５（厚さ６ｎｍ）と、ｉ型（Ｉｎ_0.52Ａｌ_0.48）_x（Ｉｎ_0.53Ｇａ_0.47）_1-xＡｓ第１井戸層６７（厚さ１０ｎｍ、ｘの値は、０から０.５までリニアに変調させる。）と、ｎ型Ｉｎ_0.52Ａｌ_0.48Ａｓ第２障壁層７１（厚さ３.５ｎｍ）と、ｉ型（Ｉｎ_0.52Ａｌ_0.48）_x（Ｉｎ_0.53Ｇａ_0.47）_1-xＡｓ第２井戸層７５（厚さ１０ｎｍ、ｘの値は、０.５から０までリニアに変調させる。）と、ｉ型Ｉｎ_0.52Ａｌ_0.48Ａｓ第３障壁層７７（厚さ６ｎｍ）と、ｎ型Ｉｎ_0.53Ｇａ_0.47Ａｓコレクタ層８１とを有しており、ｎ型Ｉｎ_0.53Ｇａ_0.47Ａｓコレクタ層８１からｎ型Ｉｎ_0.52Ａｌ_0.48Ａｓ第１障壁層６５までは、島状の領域として形成されている。ｎ型Ｉｎ_0.53Ｇａ_0.47Ａｓエミッタ層６３に対してエミッタ電極８３が、ｎ型Ｉｎ_0.53Ｇａ_0.47Ａｓコレクタ層８１に対してコレクタ電極８５が形成されている。
【００２４】
上記構造では、第２障壁層７１にｎ型の不純物がドーピングされている。第２障壁層７１にｎ型の不純物をドーピングすると、上述のように導入された部分のバンド構造が変化し、図７にエネルギーバンド図で示す構造になる。すなわち、第２障壁層７１にｎ型の不純物をドーピングすると、図７に示すように第２障壁層７１の部分のポテンシャルエネルギーが下がる。より詳細には、フェルミエネルギーＥ_Fを基準とした伝導帯端と価電子帯端との両方のバンド端のエネルギーが下がる。その結果、第１及び第２の井戸層における価電子帯端の傾斜が急になり、価電子帯とスピン・スプリット・オフ帯のエネルギー［実際には（３）式の（１／｛Ｅ_F−Ｅ_{Γ 8}｝）と（１／｛Ｅ_F−Ｅ_{Γ 7}｝）］の微分値が単なる組成変調構造の場合よりも大きくなる。従って、第１及び第２の井戸層内のスピン−軌道相互作用が大きくなった結果、それぞれのΔ_Rの絶対値も大きくなる。第２障壁層への不純物のドーピングは、原子層ドーピング（アトミック・プレーナ・ドーピング：ＡＰＤ）を用いても良い。
【００２５】
図８に、図６及び図７とに示す構造におけるそれぞれのスピン状態による電流の対電圧特性を示す。前述の図５に示す、第２障壁層がノンドープの場合の第一スピン状態にある電子による電流が最大になる電圧と、第二スピン状態にある電子による電流が最大になる電圧との差が、約１.５ｍｅＶであるのに対して、図８に示すように、第２障壁層にｎ型不純物をドーピングした場合のこれに相当する電圧の差は約１０ｍｅＶであり、分離特性が大幅に向上することがわかる。
【００２６】
以上のように、組成傾斜層でつくられた量子井戸を有する組成変調三重障壁スピンフィルター装置の構造において、組成傾斜層に挟まれた第２障壁層にｎ型不純物をドーピングすることにより価電子帯における組成傾斜層のエネルギー変化の傾きを大きくすることができ、第一スピン状態にある電子による電流が最大になる電圧と、第二スピン状態にある電子による電流が最大になる電圧との差を大きくすることができる。
【００２７】
加えて、組成傾斜層を用いずに第２の障壁層にｐ型不純物を導入する構造（ドーピングにより実現した三重障壁スピンフィルター）と比べて、エネルギーバンド構造の設計が簡単になり、精度の良い構造設計を行うことができるという利点がある。
【００２８】
以上、実施の形態に沿って本発明を説明したが、本発明はこれらに制限されるものではない。その他、種々の変更、改良、組み合わせが可能なことは当業者に自明であろう。例えば、三重障壁構造の代わりに多重障壁構造を用いても良い。
【００２９】
【発明の効果】
本発明による三重障壁スピンフィルターによれば、価電子帯における組成傾斜層のエネルギー変化の傾きを大きくすることができ、第一スピン状態にある電子による電流が最大になる電圧と、第二スピン状態にある電子による電流が最大になる電圧との差を大きくすることができる。
【００３０】
加えて、組成傾斜層を用いずに第２の障壁層にｐ型不純物を導入する構造（ドーピングにより実現した三重障壁スピンフィルター）と比べて、エネルギーバンド構造の設計が簡単になり、精度の良い構造設計を行うことができる。
【図面の簡単な説明】
【図１】組成変調を利用せず不純物ドーピングのみによって形成した半導体スピンフィルター素子の概略的な構造例を示す断面図である。
【図２】図１に示す半導体スピンフィルターのエネルギーバンド図及びスピン分離エネルギーの値を示す図である。
【図３】組成変調半導体スピンフィルター装置の構造例を示す模式的な断面図である。
【図４】図３の構造に対応するエネルギーバンド図及びスピン分離エネルギーの値を示す図である。
【図５】第２障壁層をドープしない場合の組成変調半導体スピンフィルター装置における、第一および第二スピン状態にある電子による電流−電圧特性を示す図である。
【図６】本実施の形態による半導体スピンフィルター装置の構造を示す断面図である。
【図７】図６に示す半導体スピンフィルター装置の構造のエネルギーバンド図及びスピン分離エネルギーの値を示す図である。
【図８】第２障壁層をｎ型にドープした場合の組成変調半導体スピンフィルター装置における、第一および第二スピン状態にある電子による電流−電圧特性を示す図である。
【図９】一般的な強磁性体金属から半導体２次元ガスへのスピン注入を応用したデバイスの構造を示す図である。
【符号の説明】
Ｃ…半導体スピンフィルター装置、６１…ＩｎＰ基板、６３…ｎ型Ｉｎ_0.53Ｇａ_0.47Ａｓエミッタ層、６５…ｉ型Ｉｎ_0.52Ａｌ_0.48Ａｓ第１障壁層、６７…ｉ型（Ｉｎ_0.52Ａｌ_0.48）_x（Ｉｎ_0.53Ｇａ_0.47）_1-xＡｓ第１井戸層、７１…ｎ型Ｉｎ_0.52Ａｌ_0.48Ａｓ第２障壁層、７５…ｉ型（Ｉｎ_0.52Ａｌ_0.48）_x（Ｉｎ_0.53Ｇａ_0.47）_1-xＡｓ第２井戸層、７７…ｉ型Ｉｎ_0.52Ａｌ_0.48Ａｓ第３障壁層、８１…ｎ型Ｉｎ_0.53Ｇａ_0.47Ａｓコレクタ層、８３…エミッタ電極、８５…コレクタ電極。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a spin filter that selects according to the physical properties of spins possessed by carriers and extracts carriers having a specific spin, and in particular, selectively selects carriers having a specific spin among electrons or holes in a semiconductor. The present invention relates to a semiconductor spin filter device to be extracted.
[0002]
[Prior art]
Conventional semiconductor devices mainly use the degree of freedom regarding the charge of carriers. Recently, applications to devices that use the degree of freedom regarding spin of carriers, such as spin transistors that use spin physical properties of electrons, semiconductor qubit information reading devices that use spin, and the like have been proposed. In devices using the degree of freedom of electron spin, an important elemental technology is how to create spin-polarized electrons in a solid.
[0003]
There are various prior literatures related to the above technique, but many are related to a technique using the magnetic properties of materials. For example, a technique relating to spin injection from a ferromagnetic metal into a semiconductor two-dimensional electron gas is described in M. Johnson, Phys. Rev. B. 58, 1790 (1998). FIG. 9 is a diagram showing a device structure to which spin injection from a general ferromagnetic metal into a semiconductor two-dimensional gas is applied. As shown in FIG. 9, injecting spin-polarized electrons from the ferromagnetic metal electrode 102 into the semiconductor two-dimensional electron gas 104 applies a voltage 105 to the ferromagnetic metal electrodes 102 and 103 formed on the semiconductor 101. Caused by. As a result, a spin-dependent current 106 flows. This device is a device that utilizes a phenomenon in which current or voltage changes according to the degree of spin polarization in an electronic device.
[0004]
[Problems to be solved by the invention]
However, the above technique has the following problems. That is, spin-polarized electrons injected into the semiconductor are easily affected by a magnetic field leaking from the ferromagnetic metal electrodes 102 and 103 and a magnetic field applied from the outside. When using the magnetic properties of a material, it is necessary to separate a part of the material according to the direction of spin (Zeeman separation) or to have a magnetic moment in a specific direction It is necessary to apply an external magnetic field.
[0005]
Furthermore, a spin filter using electron spin-orbit interaction has been proposed as a method that does not depend on the magnetic properties of the material. For example, resonant tunneling devices using semiconductor double barrier structures have been proposed (A. Voskoboynikov, Shiue Shin Liu and CP Lee, Phys. Rev. B 59, 12514 (1999), T. Koga, J. Nitta. , H. Takayanagi and S. Datta, Phys. Rev. Lett. 88, 126601 (2002).). The spin filtering efficiency representing the characteristics of the spin filter using the spin-orbit interaction of electrons is expressed by the following formula (1).
| I ₊ −I ₋ | / | I ₊ + I ₋ | (1)
[0006]
Here, I ₊ is a current due to electrons in the first spin state, and I ₋ is a current due to electrons in the second spin state. In the semiconductor double barrier structure, the spin filtering efficiency with respect to the total amount of electrons passing through the barrier is at most about 40%. For this reason, it is predicted that the function as a spin filter is not sufficiently exhibited in a general semiconductor double barrier structure.
[0007]
An object of this invention is to provide the spin filter which can obtain high spin filtering efficiency.
[0008]
[Means for Solving the Problems]
According to one aspect of the present invention, a first barrier layer, a second barrier layer, a third barrier layer, and a first well layer formed between the first barrier layer and the second barrier layer. A first well layer in which energy at a band edge of a valence band or a conduction band is changed by composition modulation from the first barrier layer toward the second barrier layer; the second barrier layer; and the third barrier layer. A second well layer formed between the first barrier layer and the third barrier layer, wherein the energy at the band edge of the valence band or the conduction band is changed by composition modulation from the second barrier layer toward the third barrier layer. A spin filter having a triple barrier structure in which the directions of spin-orbit interaction acting on carriers between the first well layer and the second well layer are opposite to each other, Strength of spin-orbit interaction by doping two barrier layers n-type or p-type Spin filter is provided, characterized in that it is enhanced.
[0009]
To manipulate the spin-orbit interaction (magnitude) acting on carriers in semiconductor heterostructures, the valence band and spin split-off band (spin orbit separation band) are based on the Fermi energy of electrons. The edge energy needs to be spatially changed. For example, in the above spin filter, it is possible to change the spatial change of the energy at the band edge of the valence band and the spin split-off band by doping the second barrier layer into n-type or p-type. .
[0010]
DETAILED DESCRIPTION OF THE INVENTION
In the previous application, the inventors proposed a semiconductor triple barrier structure using compositional modulation. First, the semiconductor triple barrier structure will be described with reference to the drawings. FIG. 1 is a cross-sectional view showing a schematic structural example of an element structure of a semiconductor spin filter formed only by impurity doping without using composition modulation, and FIG. 2 is an energy band diagram of the semiconductor spin filter shown in FIG. It is a figure which shows the value of spin separation energy.
[0011]
The semiconductor spin filter A shown in FIG. 1 includes, for example, an InP substrate 1, an n-type In _0.53 Ga _0.47 As collector layer 3 sequentially formed on the InP substrate 1, and an n-type In _0.52 Al _0.48 As first layer. A barrier layer 5 (thickness 6 nm), an i-type In _0.53 Ga _0.47 As first well layer 7 (thickness 10 nm), a p-type In _0.52 Al _0.48 As second barrier layer 11 (thickness 3.5 nm), An i-type In _0.53 Ga _0.47 As second well layer 15 (thickness 10 nm), an n-type In _0.52 Al _0.48 As third barrier layer 17 (thickness 6 nm), and an n-type In _0.53 Ga _0.47 As emitter layer 21 The n-type In _0.53 Ga _0.47 As emitter layer 21 to the n-type In _0.52 Al _0.48 As first barrier layer 5 are formed as island-shaped regions. A collector electrode 23 is formed for the n-type In _0.53 Ga _0.47 As collector layer 3, and an emitter electrode 25 is formed for the n-type In _0.53 Ga _0.47 As emitter layer 21. Here, i-type refers to a non-doped type. It should be noted that the structure from the collector layer 3 to the emitter layer 21 may be an inverted structure. The n-type impurity doping amount N _D is, for example, on the order of 10 ¹⁸ cm ⁻³ , and the p-type impurity doping amount N _A is, for example, about 1 × 10 ¹⁹ cm ⁻³ . As for the first barrier layer 5 and the third barrier layer 17, there is a method in which an impurity is introduced outside the barrier layer (emitter side and collector side) in order to reduce carrier impurity scattering.
[0012]
As shown in FIG. 2, since the second barrier layer 11 (FIG. 1) is doped with a p-type impurity, the energy values of the conduction band edge and the valence band edge in the second barrier layer 11 are increased, and Is realized. The spin separation energy value (Δ _R ) is also shown on the upper side of FIG. The spin separation energy value (Δ _R ) was calculated based on the following formulas (2) and (3) using conditions of E _kz = 0.2 eV and E _k ‖ = 0.1 eV.
[0013]
Δ _R = 2αk‖ (2)
[Expression 1]

[0014]
Here, k‖ is a component in the in-plane direction of wave number (a component parallel to the heterointerface), h is a Planck constant, E _p is a k · p interaction parameter, m ₀ is a free electron mass, and E _F is an electron. Fermi energy, E _{Γ 8} and E _{Γ 7} , are energy at the band edge of the valence band and the spin split-off band, respectively.
[0015]
The length of the arrow shown in FIG. 2, the energy value of the gamma ₈ relative to the Fermi energy E _F gamma _7, i.e., corresponding to the value of the E _F -E _{gamma 8} and E _F -E _{Γ 7.} Further, as shown in the equation (3), it is understood that the Rashba spin-orbit interaction coefficient α is calculated as a difference between the reciprocal derivatives of these values.
[0016]
In the triple barrier spin filter realized by doping, as shown in FIG. 2, the Rashba spin separation energy Δ _R has a relatively large value of about ± 1.5 meV to ± 2.5 meV. . The difference between the emitter-collector voltage that gives the peak value of the spin-up current obtained based on the IV characteristic and the emitter-collector voltage that gives the peak value of the spin-down current is the first and third barrier layers. When the n-type impurity doping amount N _D is 2 × 10 ¹⁸ cm ⁻³ and the p-type impurity doping amount N _A is 6.85 × 10 ¹⁸ cm ⁻³ , a value of about 7 mV is obtained. When N _D is 4 × 10 ¹⁸ cm ⁻³ and the p-type impurity doping amount N _A is 1.37 × 10 ¹⁹ cm ⁻³ , a value of about 12 mV is obtained (T. Koga, J. Nitta , and H. Takayanagi, "Spin-Filter Device Based on the Rashba Effect Using a Nonmagnetic Resonant Tunneling Diode", Physical Review Letters, Vol. 88, No. 12, 25 March 2002 pp.126601-1-126601-4.) .
[0017]
Thus, it has been theoretically shown that the triple barrier spin filter realized by doping exhibits very excellent spin separation characteristics. However, a high concentration of p-type impurity is added to the second barrier layer with good controllability. Doping is not technically easy. In addition, the band engineering itself is poorly controllable only by high-concentration impurity doping, and is easily affected by impurity scattering.
[0018]
Therefore, the inventors considered realizing a semiconductor spin filter device by composition modulation. FIG. 3 is a schematic cross-sectional view showing a structural example of a composition-modulated semiconductor spin filter device, and FIG. 4 is an energy band diagram corresponding to the structure of FIG. The semiconductor spin filter B shown in FIG. 3 includes, for example, an InP substrate 31, an n-type In _0.53 Ga _0.47 As collector layer 33 sequentially formed on the InP substrate 31, and an i-type In _0.52 Al _0.48 As first layer. The barrier layer 35 (thickness 6 nm) and the i-type (In _0.52 Al _0.48 ) _X (In _0.53 Ga _0.47 ) _1-X As first well layer 37 (thickness 10 nm, the value of x ranges from 0 to 0.5 Linearly modulated), i-type In _0.52 Al _0.48 As second barrier layer 41 (thickness 3.5 nm), i-type (In _0.52 Al _0.48 ) _X (In _0.53 Ga 0.47) _1-X As second Well layer 45 (thickness 10 nm, value of x is linearly modulated from 0.5 to 0), i-type In _0.52 Al _0.48 As third barrier layer 47 (thickness 6 nm), n-type In _0.53 has a Ga _0.47 as emitter layer 51, n-type in _0.53 Ga _0.47 as Emi From data layer 51 to i-type In _0.52 Al _0.48 As first barrier layer 35 is formed as an island-like region. A collector electrode 53 is formed for the n-type In _0.53 Ga _0.47 As collector layer 33, and an emitter electrode 55 is formed for the n-type In _0.53 Ga _0.47 As emitter layer 51.
[0019]
It should be noted that the structure from the collector layer 33 to the emitter layer 51 may be an inverted structure. Unlike the structure of FIG. 1, impurities are doped from the i-type In _0.52 Al _0.48 As first barrier layer 35 (thickness 6 nm) to the i-type In _0.52 Al _0.48 As third barrier layer 47 (thickness 6 nm). Not. Instead, in the first well layer 37 and the second well layer 45, _{x of} (In _0.52 Al _0.48 ) _x (In _0.53 Ga _0.47 ) _1-x As is changed while maintaining lattice matching with InP. Thus, the composition modulation layer is formed.
[0020]
As a result, as shown in FIG. 4, the energy Ec at the conduction band edge of the first well layer 37 and the second well layer 45 increases toward the second barrier layer 41. In addition, since the composition is inclined unlike the energy band diagram shown in FIG. 2, the band gap changes in the thickness direction in the first well layer 37 and the second well layer 45. More particularly, gradually becomes larger band gap toward the second barrier layer 41, together with the energy value of the conduction band edge relative to the Fermi energy E _F is increased, the energy value of the valence band edge is lower . In the case of composition modulation, when the second barrier layer 41 is not doped with p-type impurities, the energy value at the conduction band edge has a mountain-shaped potential structure, and the triple barrier spin filter is realized by doping. The same effect can be expected.
[0021]
FIG. 5 shows current-voltage characteristics depending on the spin of the triple barrier spin filter realized by using the composition gradient layer. As shown in FIG. 5, the difference between the voltage corresponding to the peak of the current due to the electrons in the first spin state and the voltage corresponding to the peak of the current due to the electrons in the second spin state is about 1.5 mV. It can be seen that it is considerably smaller than the structure in which the second barrier layer is doped with p-type impurities (about 12 mV). This is expressed as the differential of the reciprocal of the values of E _F −E _{Γ 8} and E _F −E _{Γ 7} , as shown in the equation (3), as the value of Rashba's spin-orbit interaction coefficient α. Because.
[0022]
Based on the above studies, the inventor considered that it is important to design not only the conduction band but also the energy band structure of the valence band in order to improve the performance as a spin filter. We have come up with a structure that further improves the performance as a spin filter, not just a structure in which a compositional gradient layer is formed in a semiconductor heterostructure. That is, when ionized n-type impurities are introduced into the semiconductor heterostructure, the band structure of the introduced region is distorted and the potential energy is lowered. By utilizing this, an n-type impurity may be introduced into the second barrier layer.
[0023]
Based on the above consideration, a semiconductor spin filter device according to an embodiment of the present invention will be described with reference to the drawings. FIG. 6 is a sectional view showing the structure of the semiconductor spin filter device according to the present embodiment. FIG. 7 is an energy band diagram of the structure of the semiconductor spin filter device shown in FIG. As shown in FIG. 6, the semiconductor spin filter device C according to the present embodiment includes, for example, an InP substrate 61, an n-type In _0.53 Ga _0.47 As emitter layer 63 sequentially formed on the InP substrate 61, and doped n-type, An i-type In _0.52 Al _0.48 As first barrier layer 65 (thickness 6 nm) and an i-type (In _0.52 Al _0.48 ) _x (In _0.53 Ga _0.47 ) _1-x As first well layer 67 (thickness 10 nm, x The value is linearly modulated from 0 to 0.5.), N-type In _0.52 Al _0.48 As second barrier layer 71 (thickness 3.5 nm), and i-type (In _0.52 Al _0.48 ) _x (In _0.53 Ga _0.47 ) _1-x As second well layer 75 (thickness 10 nm, the value of x is linearly modulated from 0.5 to 0) and i-type In _0.52 Al _0.48 As third barrier layer 77 (thickness) 6 nm) and an n-type In _0.53 Ga _0.47 As collector layer 81. Thus, the n-type In _0.53 Ga _0.47 As collector layer 81 to the n-type In _0.52 Al _0.48 As first barrier layer 65 are formed as island-shaped regions. An emitter electrode 83 is formed for the n-type In _0.53 Ga _0.47 As emitter layer 63, and a collector electrode 85 is formed for the n-type In _0.53 Ga _0.47 As collector layer 81.
[0024]
In the above structure, the second barrier layer 71 is doped with n-type impurities. When the second barrier layer 71 is doped with an n-type impurity, the band structure of the portion introduced as described above changes, and the structure shown in the energy band diagram of FIG. 7 is obtained. That is, when the second barrier layer 71 is doped with an n-type impurity, the potential energy of the second barrier layer 71 decreases as shown in FIG. More specifically, the energy of both the band edge of the Fermi energy E _F conduction band edge and valence band edge relative to the falls. As a result, the slopes of the valence band edges in the first and second well layers become steep, and the energy of the valence band and the spin split-off band [actually, (1 / {E _F −E _{Γ 8} }) and (1 / {E _F −E _{Γ 7} })] are larger than those in the case of a simple composition modulation structure. Thus, the first and second well layers in the spin - orbit interaction is increased result, the greater the absolute value of each delta _R. For doping the impurity into the second barrier layer, atomic layer doping (atomic planar doping: APD) may be used.
[0025]
FIG. 8 shows current versus voltage characteristics depending on the spin states in the structures shown in FIGS. The difference between the voltage at which the current due to electrons in the first spin state when the second barrier layer is non-doped shown in FIG. 5 and the voltage at which the current due to electrons in the second spin state is maximized is as follows. , About 1.5 meV, as shown in FIG. 8, when the second barrier layer is doped with an n-type impurity, the voltage difference corresponding to this is about 10 meV, which greatly improves the isolation characteristics. It turns out that it improves.
[0026]
As described above, in the structure of the composition modulation triple barrier spin filter device having the quantum well made of the composition gradient layer, the valence band is obtained by doping the second barrier layer sandwiched between the composition gradient layers with the n-type impurity. The gradient of the energy change of the composition gradient layer in can be increased, and the difference between the voltage at which the current due to the electrons in the first spin state is maximum and the voltage at which the current due to the electrons in the second spin state is maximum Can be bigger.
[0027]
In addition, the design of the energy band structure is simpler and more accurate than a structure in which a p-type impurity is introduced into the second barrier layer without using a composition gradient layer (a triple barrier spin filter realized by doping). There is an advantage that structural design can be performed.
[0028]
As mentioned above, although this invention was demonstrated along embodiment, this invention is not restrict | limited to these. It will be apparent to those skilled in the art that other various modifications, improvements, and combinations can be made. For example, a multiple barrier structure may be used instead of the triple barrier structure.
[0029]
【The invention's effect】
According to the triple barrier spin filter of the present invention, the gradient of the energy change of the composition gradient layer in the valence band can be increased, the voltage at which the current due to the electrons in the first spin state is maximized, and the second spin state It is possible to increase the difference from the voltage at which the current due to the electrons in the current becomes maximum.
[0030]
In addition, the design of the energy band structure is simpler and more accurate than a structure in which a p-type impurity is introduced into the second barrier layer without using a composition gradient layer (a triple barrier spin filter realized by doping). Structural design can be performed.
[Brief description of the drawings]
FIG. 1 is a cross-sectional view showing a schematic structure example of a semiconductor spin filter element formed by only impurity doping without using composition modulation.
FIG. 2 is a diagram showing an energy band diagram and a value of spin separation energy of the semiconductor spin filter shown in FIG.
FIG. 3 is a schematic cross-sectional view showing a structural example of a composition-modulated semiconductor spin filter device.
4 is a diagram showing an energy band diagram corresponding to the structure of FIG. 3 and a value of spin separation energy. FIG.
FIG. 5 is a diagram showing current-voltage characteristics due to electrons in first and second spin states in a compositionally modulated semiconductor spin filter device when the second barrier layer is not doped.
FIG. 6 is a cross-sectional view showing the structure of the semiconductor spin filter device according to the present embodiment.
7 is a diagram showing an energy band diagram and a value of spin separation energy of the structure of the semiconductor spin filter device shown in FIG. 6. FIG.
FIG. 8 is a diagram showing current-voltage characteristics due to electrons in first and second spin states in a compositionally modulated semiconductor spin filter device when the second barrier layer is doped n-type.
FIG. 9 is a diagram showing the structure of a device using spin injection from a general ferromagnetic metal to a semiconductor two-dimensional gas.
[Explanation of symbols]
C ... Semiconductor spin filter device, 61 ... InP substrate, 63 ... n-type In _0.53 Ga _0.47 As emitter layer, 65 ... i-type In _0.52 Al _0.48 As first barrier layer, 67 ... i-type (In _0.52 Al _0.48 ) _x ( In _0.53 Ga _0.47 ) _1-x As first well layer, 71... N-type In _0.52 Al _0.48 As second barrier layer, 75... I-type (In _0.52 Al _0.48 ) _x (In _0.53 Ga _0.47 ) _1-x As second 2-well layer, 77 ... i-type In _0.52 Al _0.48 As third barrier layer, 81 ... n-type In _0.53 Ga _0.47 As collector layer, 83 ... emitter electrode, 85 ... collector electrode.

Claims (4)

A first barrier layer, a second barrier layer, a third barrier layer, and a first well layer formed between the first barrier layer and the second barrier layer; Formed between the first well layer in which the energy at the band edge of the valence band or the conduction band is changed by composition modulation toward the second barrier layer, and the second barrier layer and the third barrier layer. A second well layer, wherein the energy of the band edge of the valence band or the conduction band is changed by composition modulation from the second barrier layer toward the third barrier layer, and A spin filter having a triple barrier structure in which the direction of spin-orbit interaction acting on carriers between the first well layer and the second well layer is opposite,
A spin filter having a spin-orbit interaction enhanced by doping the second barrier layer to n-type or p-type. An InP substrate;
An n-type emitter layer, an i-type In _0.52 Al _0.48 As first barrier layer, and an i-type (In _0.52 Al _0.48 ) _x (In _0.53 Ga _0.47 ) _1-x As first well layer formed on the InP substrate. A first well layer in which the value of x is changed from 0 to x _max (0 <x _max ≦ 1), an n-type In _0.52 Al _0.48 As second barrier layer, and an i-type (In _0.52 Al _0.48 ) _X (In _0.53 Ga _0.47 ) _1-X As second well layer, wherein the value of x is modulated from x ′ _max (0 <x ′ _max ≦ 1) to 0; A spin-orbit interaction acting on carriers of the first conductivity type between the first well layer and the second well layer, comprising a third barrier layer of type In _0.52 Al _0.48 As and an n-type collector layer. A spin filter having a triple barrier structure with opposite directions. An InP substrate;
A p-type emitter layer, an i-type In _0.52 Al _0.48 As first barrier layer, and an i-type (In _0.52 Al _0.48 ) _x (In _0.53 Ga _0.47 ) _1-x As first well layer formed on the InP substrate. A first well layer in which the value of x is changed from 0 to x _max (0 <x _max ≦ 1), a p-type or n-type doped In _0.52 Al _0.48 As second barrier layer, and i Type (In _0.52 Al _0.48 ) _X (In _0.53 Ga _0.47 ) _1-X As second well layer, in which the value of x is modulated from x ′ _max (0 <x ′ _max ≦ 1) to 0 A spin well including a two well layer, an i-type In _0.52 Al _0.48 As third barrier layer, and a p-type collector layer, and acting on carriers of the second conductivity type in the first well layer and the second well layer; A spin filter having a triple barrier structure in which the direction of orbital interaction is opposite. 4. The spin filter according to claim 1, wherein an impurity is introduced into the second barrier layer by atomic planar doping. 5.

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